The Cambridge natural history, Vol. 07 (of 10)

THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, Sc.D., F.R.S., Fellow of King's College, Cambridge; Superintendent of the University Museum of Zoology

AND

A. E. SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge; University Lecturer on the Morphology of Invertebrates

VOLUME VII

HEMICHORDATA

By S. F. Harmer, Sc.D., F.R.S., Fellow of King's College, Cambridge.

ASCIDIANS AND AMPHIOXUS

By W. A. Herdman, D.Sc. (Edinb.), F.R.S., Professor of Natural History in the University of Liverpool.

FISHES (Exclusive of the Systematic Account of Teleostei)

By T. W. Bridge, Sc.D., F.R.S., Trinity College, Cambridge; Mason Professor of Zoology and Comparative Anatomy in the University of Birmingham.

FISHES (Systematic Account of Teleostei)

By G. A. Boulenger, F.R.S., of the British Museum (Natural History).

London
MACMILLAN AND CO., Limited
NEW YORK: THE MACMILLAN COMPANY
1904

All rights reserved

Third Fisherman.—Master, I marvel how the fishes live in the sea.

First Fisherman.—Why, as men do a-land,—the great ones eat up the little ones.

Pericles, Act II. Scene i.

PREFACE

Owing to unforeseen circumstances, not unconnected with the foundation of a new University, the publication of this volume has been unduly delayed. Some parts of the work have actually been in type for more than four years; and although the authors have made every effort to keep them up to date, the arrangement is naturally not quite what it might have been if the articles had been written immediately before publication.

In view of the novelty of Mr. Boulenger's classification of the Teleosteans, and of the fact that several independent workers have been occupying themselves with the subject during the last year or two, it is fair to state that this part of the volume was completed in 1902. Professor Herdman's account of the Ascidians was ready for publication two years earlier.

Professor Bridge wishes to express his best thanks to Dr. R. H. Traquair. F.R.S., for his kindness in reading the proofs of the pages which deal with the fossil Crossopterygii, Chondrostei, Holostei and Dipnoi, and for much helpful advice and criticism; to Mr. G. A. Boulenger, F.R.S., for his valuable and suggestive criticism on certain points; and to Mr. Edwin Wilson, for the care which he has taken in the preparation of the figures.

July 1904.

CONTENTS

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

The names of extinct groups are printed in italics.

CHORDATA (p. [3]).
I. HEMICHORDATA (p. [3]).
Order.						Family.
ENTEROPNEUSTA (p. [5])						Glandicipitidae (p. [17]). Ptychoderidae (p. [17]). Harrimaniidae (p. [17]).
PTEROBRANCHIA (p. [21])
PHORONIDEA (p. [27])
II. UROCHORDATA = TUNICATA (pp. [4], [35], [63]).
Order.		Sub-Order.				Family.		Sub-Family.
LARVACEA (p. [64])						Kowalevskiidae (p. [68]). Appendiculariidae (p. [68]).
ASCIDIACEA (p. [70])		Ascidiae Simplices (p. [71]).				Clavelinidae (p. [71]).
						Ascidiidae (p. [72]).		Hypobythiinae (p. [72]). Ascidiinae (p. [72]). Corellinae (p. [73]).
						Cynthiidae (p. [74]).		Styelinae (p. [74]). Cynthiinae (p. [75]). Bolteninae (p. [75]).
						Molgulidae (p. [77]).
		Ascidiae Compositae (p. [80])		Merosomata (p. [85])		Distomatidae (p. [85]). Coelocormidae (p. [86]). Didemnidae (p. [86]). Diplosomatidae (p. [87]). Polyclinidae (p. [87]).
				Holosomata (p. [88])		Botryllidae (p. [88]). Polystyelidae (p. [89]).
		Ascidiae Luciae (p. [90])				Pyrosomatidae (p. [91]).
THALIACEA (p. [95])		Cyclomyaria (p. [95]).				Doliolidae (p. [96]).
		Hemimyaria (p. [101]).				Salpidae (p. [101]). Octacnemidae (p. [108]).
III. CEPHALOCHORDATA (pp. [4], [112]).
						Family.
						Branchiostomatidae (p. [137]).
IV. CRANIATA (pp. [4], [141]).
Class—Cyclostomata (pp. [145], [150], [421]).
Sub-Class.		Order.		Sub-Order.		Family.		Sub-Family.
		Myxinoides (p. [421])				Myxinidae (p. [422]). Bdellostomatidae (p. [423]).
		Petromyzontes (p. [425])				Petromyzontidae (p. [426]).
Class—Pisces (pp. [145], [431]).
ELASMOBRANCHII (p. [431])		Pleuropterygii (p. [436])				Cladoselachidae (p. [438]).
		Ichthyotomi (p. [440])				Pleuracanthidae (p. [440]).
		Acanthodei (p. [440])				Diplacanthidae (p. [441]). Acanthodidae (p. [441]).
		Plagiostomi (p. [442])		Selachii (p. [442])		Notidanidae (p. [442]). Chlamydoselachidae (p. [443]). Heterodontidae (p. [444]). Cochliodontidae (p. [445]). Psammodontidae (p. [446]). Petalodontidae (p. [446]). Scylliidae (p. [446]). Carchariidae (p. [448]). Sphyrnidae (p. [449]). Lamnidae (p. [450]). Cetorhinidae (p. [453]). Rhinodontidae (p. [454]). Spinacidae (p. [454]). Rhinidae (p. [456]). Pristiophoridae (p. [457]).
				Batoidei (p. [457])		Pristidae (p. [459]). Rhinobatidae (p. [460]). Raiidae (p. [461]). Tamiobatidae (p. [462]). Torpedinidae (p. [462]). Trygonidae (p. [464]). Myliobatidae (p. [465]).
		Holocephali (p. [466])				Ptyctodontidae (p. [468]). Squaloraiidae (p. [468]). Myriacanthidae (p. [468]). Chimaeridae (p. [468]).
TELEOSTOMI (p. [475])		Crossopterygii (p. [476])		Osteolepida (p. [437])		Osteolepidae (p. [477]). Rhizodontidae (p. [478]). Holoptychidae (p. [479]). Coelacanthidae (p. [480]).
				Cladistia (p. [481])		Polypteridae (p. [481]).
		Chondrostei (p. [485])				Palaeoniscidae (p. [486]). Platysomidae (p. [487]). Belonorhynchidae (p. [488]). Catopteridae (p. [488]). Chondrosteidae (p. [489]). Polyodontidae (p. [491]). Acipenseridae (p. [492]).
		Holostei (p. [495])				Semionotidae (p. [497]). Macrosemiidae (p. [498]). Pycnodontidae (p. [498]). Eugnathidae (p. [498]). Amiidae (p. [499]). Pachycormidae (p. [501]). Aspidorhynchidae (p. [502]). Lepidosteidae (p. [502]).
		Teleostei (pp. [504], [541])		Malacopterygii (p. [543])		Pholidophoridae (p. [545]). Archaeomaenidae (p. [545]). Oligopleuridae (p. [545]). Leptolepididae (p. [546]). Elopidae (p. [546]). Albulidae (p. [547]).
						Mormyridae (p. [549])		Mormyrinae (p. [551]). Gymnarchinae (p. [551])
						Hyodontidae (p. [552]). Notopteridae (p. [554]). Osteoglossidae (p. [555]). Pantodontidae (p. [558]). Ctenothrissidae (p. [559]). Phractolaemidae (p. [560]). Saurodontidae (p. [561]). Chirocentridae (p. [561]).
						Clupeidae (p. [562])		Thrissopatrinae (p. [562]). Engraulinae (p. [563]). Clupeinae (p. [563]). Chaninae (p. [563]).
						Salmonidae (p. [565]). [Pachyrhizodontidae (p. [569]).] Alepocephalidae (p. [569]).
						Stomiatidae (p. [570])		Chauliodontinae (p. [571]). Sternoptychinae (p. [571]). Stomiatinae (p. [571]).
						Gonorhynchidae (p. [572]). Cromeriidae (p. [573]).
				Ostariophysi (p. [573])		Characinidae (p. [575])		Erythrininae (p. [575]). Hydrocyoninae (p. [575]). Serrasalmoninae (p. [576]). Ichthyoborinae (p. [576]). Xiphostominae (p. [576]). Anostominae (p. [576]). Hemiodontinae (p. [576]). Distichodontinae (p. [576]). Citharininae (p. [576]).
						Gymnotidae (p. [579]).
						Cyprinidae (p. [581])		Catostominae (p. [581]). Cyprininae (p. [582]). Cobitidinae (p. [582]). Homalopterinae (p. [582]).
						Siluridae (p. [586])		Clariinae (p. [588]). Silurinae (p. [588]). Bagrinae (p. [588]). Doradinae (p. [588]). Malopterurinae (p. [588]). Callichthyinae (p. [588]). Hypophthalminae (p. [589]). Trichomycterinae (p. [589]).
						Loricariidae (p. [594])		Arginae (p. [595]). Loricariinae (p. [595]).
						Aspredinidae (p. [596]).
				Symbranchii (p. [597])		Symbranchidae (p. [597]). Amphipnoidae (p. [598]).
				Apodes (p. [599])		Anguillidae (p. [600]). Nemichthyidae (p. [603]). Synaphobranchidae (p. [603]). Saccopharyngidae (p. [603]). Muraenidae (p. [604]).
				Haplomi (p. [605])		Galaxiidae (p. [607]). Haplochitonidae (p. [608]). Enchodontidae (p. [608]). Esocidae (p. [609]). Dalliidae (p. [610]). Scopelidae (p. [611]). Alepidosauridae (p. [614]). Cetomimidae (p. [614]). Chirothricidae (p. [615]). Kneriidae (p. [615]). Cyprinodontidae (p. [616]). Amblyopsidae (p. [618]). Stephanoberycidae (p. [619]). Percopsidae (p. [620]).
				Heteromi (p. [621])		Dercetidae (p. [623]). Halosauridae (p. [623]). Lipogenyidae (p. [624]). Notacanthidae (p. [624]). Fierasferidae (p. [625]).
				Catosteomi (p. [626])		Gastrosteidae (p. [629]) Aulorhynchidae (p. [631]) Protosyngnathidae (p. [631]) Aulostomatidae (p. [632]) Fistulariidae (p. [632]) Centriscidae (p. [633]) Amphisilidae (p. [633])		= Hemibranchii (p. [627]).
						Solenostomidae (p. [633]) Syngnathidae (p. [634])		= Lophobranchii (p. [628]).
						Pegasidae (p. [635])		= Hypostomides (p. [628]).
				Percesoces (p. [636])		Scombresocidae (p. [637]). Ammodytidae (p. [639]). Atherinidae (p. [639]). Mugilidae (p. [640]). Polynemidae (p. [640]). Chiasmodontidae (p. [641]). Sphyraenidae (p. [642]). Tetragonuridae (p. [642]). Stromateidae (p. [643]). Icosteidae (p. [644]). Ophiocephalidae (p. [644]). Anabantidae (p. [645]).
				Anacanthini (p. [646])		Macruridae (p. [647]). Gadidae (p. [647]). Muraenolepididae (p. [649]).
				Sub-Order.		Division.		Family.
				Acanthopterygii (p. [650])		Perciformes (p. [652])		Berycidae (p. [655]). Monocentridae (p. [656]). Pempheridae (p. [656]). Centrarchidae (p. [657]). Cyphosidae (p. [657]). Lobotidae (p. [658]). Toxotidae (p. [658]). Nandidae (p. [658]). Percidae (p. [658]). Acropomatidae (p. [659]). Serranidae (p. [659]) Subfamilies: * Serraninae (p. [659]). * Grammistinae (p. [660]). * Priacanthinae (p. [660]). * Centropominae (p. [660]). * Pomatominae (p. [660]). * Ambassinae (p. [660]). * Chilodipterinae (p. [660]). * Lutjaninae (p. [660]). * Cirrhitinae (p. [660]). * Pentacerotinae (p. [660]). Anomalopidae (p. [660]). Pseudochromididae (p. [661]). Cepolidae (p. [661]). Hoplognathidae (p. [662]). Sillaginidae (p. [662]). Sciaenidae (p. [663]). Gerridae (p. [663]). Lactariidae (p. [663]). Trichodontidae (p. [663]). Latrididae (p. [663]). Haplodactylidae (p. [664]). Pristipomatidae (p. [664]). Sparidae (p. [664]). Mullidae (p. [665]). Scorpididae (p. [666]). Caproidae (p. [666]). Chaetodontidae (p. [667]). Drepanidae (p. [668]). Acanthuridae (p. [668]). Teuthididae (p. [668]). Osphromenidae (p. [669]). Embiotocidae (p. [670]). Cichlidae (p. [670]). Pomacentridae (p. [672]). Labridae (p. [673]). Scaridae (p. [674]).
						Scombriformes (p. [675])		Carangidae (p. [677]). Rhachicentridae (p. [677]). Scombridae (p. [678]). Trichiuridae (p. [679]). Histiophoridae (p. [679]). Palaeorhynchidae (p. [680]). Xiphiidae (p. [681]). Luvaridae (p. [681]). Coryphaenidae (p. [681]). Bramidae (p. [682]).
						Zeorhombi (p. [682])		Zeidae (p. [683]). Amphistiidae (p. [684]). Pleuronectidae (p. [684]).
						Kurtiformes (p. [687])		Kurtidae (p. [687]).
						Gobiiformes (p. [688])		Gobiidae (p. [689]).
						Discocephali (p. [691])		Echeneididae (p. [691]).
						Scleroparei (p. [692])		Scorpaenidae (p. [694]). Hexagrammidae (p. [696]). Comephoridae (p. [696]). Rhamphocottidae (p. [697]). Cottidae (p. [697]). Cyclopteridae (p. [698]). Platycephalidae (p. [699]). Hoplichthyidae (p. [699]). Agonidae (p. [700]). Triglidae (p. [700]). Dactylopteridae (p. [701]).
						Jugulares (p. [702])		Trachinidae (p. [704]). Percophiidae (p. [705]). Leptoscopidae (p. [705]). Nototheniidae (p. [705]). Uranoscopidae (p. [706]). Trichonotidae (p. [706]). Callionymidae (p. [706]). Gobiesocidae (p. [707]). Blenniidae (p. [709]). Batrachidae (p. [710]). Pholididae (p. [711]). Zoarcidae (p. [712]). Congrogadidae (p. [713]). Ophidiidae (p. [713]). Podatelidae (p. [713]).
						Taeniosomi (p. [714])		Trachypteridae (p. [715]). Lophotidae (p. [716]).
				Opisthomi (p. [716])				Mastacembelidae (p. [716]).
				Pediculati (p. [717])				Lophiidae (p. [718]). Ceratiidae (p. [719]). Antennariidae (p. [720]). Gigantactinidae (p. [720]). Malthidae (p. [720]).
				Plectognathi (p. [721])		Sclerodermi (p. [722])		Triacanthidae (p. [722]). Triodontidae (p. [723]). Balistidae (p. [723]). Ostraconiidae (p. [722]).
						Gymnodontes (p. [725])		Tetrodontidae (p. [726]). Diodontidae (p. [726]). Molidae (p. [726]).
DIPNEUSTI = DIPNOI (p. [505])								Ctenodontidae (p. [505]). Uronemidae (p. [507]). Ceratodontidae (p. [507]). Lepidosirenidae (p. [511]).

OF UNCERTAIN POSITION
		Order.		Family.
Palaeospondylidae (p. [521]).
Ostracodermi (p. [522])		Heterostraci (p. [524])		Coelolepidae (p. [524]). Drepanaspidae (p. [525]). Psammosteidae (p. [526]). Pteraspidae (p. [527]).
		Osteostraci (p. [527])		Ateleaspidae (p. [528]). Cephalaspidae (p. [528]). Tremataspidae (p. [530]).
		Anaspida (p. [531])		Birkeniidae (p. [531]).
Antiarchi (p. [532])				Asterolepidae (p. [534]).
Arthrodira (p. [535])				Coccosteidae (p. [536]).

HEMICHORDATA

BY

SIDNEY F. HARMER, Sc.D., F.R.S.

Fellow of King's College, Cambridge.

CHAPTER I

HEMICHORDATA

CHORDATA AND VERTEBRATA—HEMICHORDATA—ENTEROPNEUSTA—EXTERNAL CHARACTERS AND HABITS—STRUCTURE—GENERA—DEVELOPMENT—PTEROBRANCHIA—CEPHALODISCUS AND RHABDOPLEURA—PHORONIDEA—PHORONIS AND ACTINOTROCHA—AFFINITIES OF THE HEMICHORDATA.

The Hemichordata, a marine group which includes the worm-like Balanoglossus, owe much of their interest to the fact that they are believed by many zoologists to be related to the lower Vertebrates. This view is one of a number of mutually exclusive hypotheses, which seek to derive Vertebrate animals from various Invertebrate ancestors. It is supported by many striking resemblances between Balanoglossus and the lowest forms which are by common consent regarded as belonging to the Vertebrate alliance; but it must be distinctly understood that Balanoglossus is at most the much modified modern representative of extinct forms which were also the ancestors of Vertebrates.

The axis of the backbone of all Vertebrates is formed by an elastic rod known as the "notochord" (Figs. 72, 115), which lasts throughout life in some of the lowest forms, but in the higher forms appears only in the embryo. The universal occurrence of this structure has been regarded as the most important characteristic of the Vertebrata and their allies, which are accordingly grouped together in the Phylum CHORDATA. The members of this Phylum are further distinguished from other animals by several important features. Of these one of the most important appears to be the existence of lateral outgrowths of the pharynx, which unite with the skin of the neck and form a series of perforations leading to the exterior. These structures are the gill-slits, and in the Fishes their walls give rise to vascular folds or gills. With the assumption of a terrestrial life, the higher Vertebrates lost their gills as functional organs, respiration being then performed by entirely different organs, the lungs. But even in these cases, the gill-slits appear in the embryo; and remains of one pair can usually be recognised in the adult state of even the highest Vertebrates. Another fundamental characteristic of the Chordata is given by the central nervous system, which lies entirely above the alimentary canal, just dorsal to the notochord. Not only does this position of the nerve-centres distinguish the Chordata from Invertebrates, but a further point of difference is found in the development. While in Invertebrates the ventral nerve-cord is formed as a thickening of the ectoderm or outermost layer of the embryo, in the Chordata the nervous system is usually formed as a longitudinal groove running medianly along the back of the embryo. This groove closes to form a tube of nervous matter, the cavity of which always persists throughout life as the "central canal" of the spinal chord and its anterior prolongation which constitutes the "ventricles" of the brain.

Although the animals which are considered in this chapter are not admitted by all zoologists to be related to the Vertebrates, there can be no question that their respiratory organs closely resemble typical gill-slits. Since, moreover, they possess structures which can be regarded, with a fair amount of probability, as agreeing in essential respects with the notochord and the tubular dorsal nervous system of Vertebrates, it appears justifiable to include them in the Chordata, which are then subdivided into (1) Hemichordata, in which a "notochord" occurs in the anterior end of the body only; (2) Urochordata (Tunicata or Ascidians), in which the notochord is restricted to the tail; (3) Cephalochordata (Amphioxus), in which the notochord extends the entire length of the body and of the head; (4) Craniata, in which a brain is developed as an enlargement of the central nervous system, the notochord does not extend farther forward than the middle of the brain, and a vertebral column is present. These last are thus usually known as Vertebrata, although in distinguishing an "Invertebrate" from a "Vertebrate" it is more logical to regard all Chordata as Vertebrates, since the Invertebrata are in no sense a natural group with common characteristics, their union under one name merely implying that they have no close affinity to the Vertebrates. It is often convenient in practice to divide animals into Vertebrates and Invertebrates, but from a zoological point of view a division of the animal kingdom into Molluscs and Non-Molluscs would have as much or as little significance.

The sub-phylum Hemichordata[^[1]] consists of the Orders:—(I.) Enteropneusta,[^[2]] including Balanoglossus (Fig. 1); (II.) Pterobranchia,[^[3]] represented by the genera Cephalodiscus (Fig. 9) and Rhabdopleura (Fig. 12). To these should possibly be added (III.) Phoronidea, for the reception of Phoronis (Fig. 13).

Order I. Enteropneusta.

Worm-like Hemichordata, with numerous gill-slits, a straight intestine, and a terminal anus. Proboscis separated by a narrow stalk from the simple ring-shaped collar, which is succeeded by an elongated trunk.

The structure of Balanoglossus, formerly the sole genus belonging to this Order, but now divided[^[4]] into the genera Ptychodera, Balanoglossus, Glossobalanus, Glandiceps, Spengelia, Schizocardium, Harrimania, Dolichoglossus, and Stereobalanus, has of recent years formed the subject of elaborate investigations by Spengel,[^[5]] Bateson,[^[6]] and Willey.[^[7]] More than thirty species are known, ranging in length from 25 mm.[^[8]] (Pt. bahamensis) to 2500 mm. (B. gigas), and for the most part inhabiting shallow water; Glossobalanus sarniensis occurring between tide-marks in the Channel Islands. Glandiceps talaboti has, however, been dredged near Marseilles from as much as 190 fathoms, while G. abyssicola was found by the "Challenger" at a depth of 2500 fathoms, off the West Coast of Africa.

Fig. 1.—Forms of Balanoglossus. A, Balanoglossus clavigerus, Eschsch., Naples, × ½; B, Glandiceps hacksi, Mar. (incomplete), Japan, × 1; C, Schizocardium brasiliense, Speng., Rio de Janeiro, × 1; D, Dolichoglossus kowalevskii, A.Ag., Chesapeake Bay, × 1. a, Anus; ab, abdominal and caudal regions; b, branchial region; c, collar; g, genital region; g.p, gill-pore; g.w, genital wing; h, hepatic region; m, position of mouth; p, proboscis; t, trunk. (A, B, and C from Spengel; D from Bateson.)

Balanoglossus, the largest genus now recognised by Spengel, appears to be practically world-wide in its distribution; Schizocardium is recorded from both sides of S. America; Glandiceps from the Atlantic, the Mediterranean, Japan, and the Indian Ocean; Spengelia from the South Pacific; and other species from the White Sea to New Zealand. The habitat is usually sand or gravelly sand, in which the animal forms a kind of tube by means of the abundant mucus secreted by its skin. Dolichoglossus kowalevskii (Fig. 1, D), according to Bateson,[^[9]] lives between tide-marks at a depth of about eight inches. The greater part of the body is coiled in an even, cork-screw-like spiral, while the anterior end, including the front part of the branchial region, is maintained in a vertical position. The posterior end is also kept upright, and can be moved up and down in a vertical shaft opening on the surface, thus enabling the animal to eject the undigested sand from its anus.

The coloration of Balanoglossus is often brilliant. That of D. kowalevskii[^[10]] is as follows:—The "proboscis" (cf. Fig. 1, B, p) is yellowish white; the "collar" (c) is brilliant red-orange (especially in males), with a white ring posteriorly; the "trunk" (t), the subdivision of which into "branchial," "genital," "hepatic," "abdominal," and "caudal" regions is better indicated in other species (Fig. 1, A, b, g, h, ab), is orange-yellow, shading to green-yellow in the semi-transparent caudal region. The genital region is grey in females and yellow in males, a sexual difference in colour being common in Enteropneusta. The hepatic papillae of other species may be bright green.

The odour of D. kowalevskii resembles that of "chloride of lime with a faecal admixture," while that of Balanoglossus aurantiacus suggests iodoform. All Enteropneusta are said to have a more or less offensive smell. A species of Balanoglossus is known to be intensely phosphorescent.[^[11]]

The mouth (Fig. 7, m) is situated on the ventral side, at the base of the proboscis, and is concealed by the free anterior edge of the collar, which encircles the thin "proboscis-stalk" (Fig. 3, p.s). The animal has the singular peculiarity of being unable to close its mouth;[^[12]] and thus, as it burrows through the ground, the sand which passes into the alimentary canal leaves it in a continuous column through the terminal anus.[^[13]] The large coiled "castings" formed in this way between tide-marks enable the experienced collector to infer the presence of Balanoglossus; and in a West Indian species described by Willey[^[14]] they are so large as to form "an important feature in the landscape at low tide."

The principal agents in burrowing are the proboscis and collar. An animal observed by Spengel pushed the tip of its proboscis into the sand, waves of muscular contraction meanwhile passing over the surface of the proboscis. At first the animal made slow progress; but the collar, becoming surrounded by sand, soon became a point of resistance by means of which the proboscis could bury itself yet more deeply. The animal quickly disappeared as soon as the first two regions of its body were engaged in the task of burrowing[^[15]]

This action is due partly to the muscles of the body-wall, but largely to the power possessed by the proboscis and collar of becoming swollen and turgid. Spengel has observed that these parts become flaccid when the animal is taken out of water, and can only swell again when it is replaced therein; and it may thus fairly be concluded that the enlargement is due to the taking in of water. This is probably in fact the most important function of the "proboscis-pore" and of the "collar-pores" which are described below.

Fig. 2.—Diagram of a dorsal view of a Balanoglossus-embryo, after the formation of the body-cavities, a, Alimentary canal; b.c¹, body-cavity of the proboscis; b.c², of the collar; b.c³, of the trunk. (From Bateson.)

Body-Cavities.—The existence of five separate body-cavities (Fig. 2) is one of the most fundamental facts in the anatomy of Balanoglossus. The first body-cavity, or cavity of the proboscis (b.c¹), is single and unpaired; the second body-cavities (b.c²) are paired spaces, one belonging to each side of the collar; the third body-cavities (b.c³) are similarly paired, and correspond with the trunk. While there is no connection between successive body-cavities, there are in certain regions communications between the two cavities of the same pair. Each of the paired cavities is at one time a closed lateral space between the skin and the alimentary canal. As the two spaces which constitute the pair grow towards one another, both above and below the alimentary canal, they come into such close apposition that they remain separated only by their conjoined walls. In this way are formed the dorsal and ventral mesenteries (Fig. 4, d.m, v), the former being the only one to persist in the higher Vertebrates. The body-cavities of the adult become to a large extent disguised by being traversed by connective tissue and muscles.

The hinder part of the proboscis-cavity is divided by the forward growth of the notochord (Fig. 3, n) into dorsal and ventral portions. The dorsal cavity in extending backwards becomes further subdivided into right and left halves, the latter typically opening dorsally to the exterior on the proboscis-stalk by an asymmetrical "proboscis-pore" (p.p.). Two symmetrical proboscis-pores may, however, occur, or a median pore connected with the left division of the proboscis-cavity. These may be individual variations within the limits of a single species, or may occur as a normal feature of a species.

Fig. 3.—Dorsal view of the anterior end of the body of Dolichoglossus kowalevskii, × 3. c, Collar; c.n, circular nerve; c.p, collar-pore; d, dorsal nerve; g, gill-pore; n, notochord; n.s, central nervous system, showing the anterior and posterior neuropores; p, proboscis; p.p, proboscis-pore; p.s, proboscis-stalk; t, trunk; v, ventral nerve. The nerve-plexus of the proboscis is represented as a black line. (After Bateson.)

The collar-cavities open by two "collar-pores" (Fig. 3, c.p.), situated at the posterior end of the collar, into the first pair of gill-pouches, near their external opening. Willey has recently described[^[16]] vestigial pores in relation with the "perihaemal spaces," a pair of dorsally situated outgrowths of the third body-cavities into the collar-region. Narrow "peripharyngeal spaces," also a forward growth of the third body-cavities, closely invest the pharynx in some species.

Body-Wall and Nervous System.—The body-wall (Fig. 4) consists externally of a thick ciliated epidermis (e), containing numerous gland-cells which secrete an abundant mucus. Beneath the epidermis is a basement-membrane, while more internally are layers of muscles, whose arrangement differs in different parts of the body and in different species.

The nervous system consists of a plexus of cells and fibres which lie in the basal part of the epidermis of all parts of the animal, outside the basement-membrane; the thicker portions of the plexus forming definite nerve-tracts. This intimate connexion between the epidermis and the nervous system is usually restricted to embryonic life in other animals.

Fig. 4.—Ptychodera bahamensis, Bahama Is. Transverse section through the branchial region. b, Branchial part of pharynx; b.c³, third body-cavity; d.m, dorsal mesentery; d.n, dorsal nerve; d.v, dorsal vessel; e, epidermis, with nerve-layer (black) at its base; g, genital wing; g.p, gill-pore, encroached on by the tongue-bar (t); l, lateral septum; m, longitudinal muscles; o, oesophageal or alimentary part of pharynx; r, reproductive organ; t, tongue-bar; v, ventral mesentery and ventral vessel; v.n, ventral nerve. (After Spengel.)

The main nerves of Balanoglossus are a dorsal and a ventral tract in the trunk region (Fig. 4, d.n, v.n), a circular tract (Fig. 3, c.n) connecting these two at the posterior edge of the collar, and a strong concentration of nerve-tissue round the whole of the proboscis-stalk, and of the posterior end of the proboscis (Fig. 3). In the region of the collar the nervous system attains its highest development, taking the form of a median cord passing above the alimentary canal. This cord, known as the central nervous system (Fig. 7, n.s), runs through the cavity of the collar, but is connected with the epidermis at each end. It thus becomes continuous in front with the nerve-layer on the proboscis-stalk, while posteriorly it passes into the dorsal and the circular nerve-tracts. In nearly all cases the epidermis is pushed into the cord at the points where it passes into the skin, in the form of an anterior and a posterior "neuropore" (Fig. 3). A transverse section through the extreme front or hind end of the collar accordingly shows a tubular nervous system. In certain species, as in Glossobalanus sarniensis and Ptychodera flava, a central canal, opening in front and behind, exists throughout the entire length of the central nervous system, while in G. minutus a canal of this kind occurs in the young animal, but not in the adult. The central nervous system is developed as a longitudinal dorsal groove in the larva,[^[17]] and in a similar manner in the collar which is formed as the result of regeneration after injury.[^[18]] Balanoglossus is thus typically provided with a dorsal, tubular, central nervous system, and although this arrangement does not extend beyond the limits of the collar, it shows a noteworthy resemblance to Vertebrate animals.

In some cases the central nervous system is connected with the dorsal epidermis by a varying number (1-17) of median "roots," which have been compared by Bateson with the dorsal roots of the spinal nerves of Amphioxus, and are probably remains of the embryonic connexion of the collar nervous system with the dorsal epidermis.

Alimentary Canal.—The mouth (Fig. 7, m) leads widely into the alimentary canal, which, passing through the collar, enters the branchial region, where it is characterised by the existence of communications with the exterior. These, the gill-slits, are developed, as in Vertebrates, as paired outgrowths of the alimentary canal, and new gill-slits are constantly being formed at the posterior end of the branchial region with advancing age. The maximum number of the gill-slits, and the extent of the branchial region, are by no means uniform throughout the Enteropneusta. Thus Dolichoglossus otagoensis is said to have no more than 12 pairs, Glossobalanus minutus only 40 pairs, while Balanoglossus aurantiacus may have as many as 700 pairs. In Ptychodera flava the variation is so great that Willey distinguishes[^[19]] two extreme conditions as "macrobranchiate" and "brachybranchiate" respectively, although intermediate conditions are also found. It should be noted that Balanoglossus agrees with Amphioxus in the indefinite number of the gill-slits.

The gill-slits usually have the form of the so-called "branchial pouches" or "gill-sacs" (Figs. 5, 6, g.s). Each ordinarily opens to the exterior by a small pore (Fig. 1, D, 5, g.p) or slit, situated on the dorsal side, in a shallow longitudinal groove not far from the middle line. The gill-sac has a complete wall of its own, and lies between the alimentary canal and the body-wall, communicating with the former by a U-shaped slit. While a dorsal view of the animal thus shows a linear series of simple pores, a view of the pharynx from the inside appears as in Fig. 5.

At the hind end of the pharynx the inner opening of the developing gill-sac is circular. Slightly further forward the dorsal side of the pore is indented into a crescent, which grows longer in a dorso-ventral direction, and becomes a U, whose two limbs are nearly separated by a mass of tissue, the so-called "tongue-bar" (Fig. 5, t). The special interest of this mode of development is that it is identical with what occurs in Amphioxus (p. 120), which is universally admitted to belong to the Chordata.

The gill-sacs of Balanoglossus follow one another closely, the hind wall of one being in contact with the front wall of the next, and constituting a "branchial septum" (b.s). Both septa and tongue-bars are supported by chitinous rods, which are special thickenings of the membrane at the base of their epithelium. Two rods occur in each tongue-bar, separated by an interval of body-cavity (Figs. 5, 6), and only one rod in each septum. Originally of this form—∩∩ ∩∩—the rods have joined in pairs, the united limbs forming the single rod of each branchial septum. In this respect again we have a similarity between Balanoglossus and Amphioxus, except that in the latter the concrescence proceeds one step farther, and the two rods of the tongue-bar unite, like those of the branchial septum. The latter, the so-called "primary" skeletal rods of Amphioxus, are forked ventrally as in Balanoglossus (Fig. 5).

Fig. 5.—Diagram of two gill-sacs of Balanoglossus, seen from the inside of the pharynx. b, Branchial skeleton, consisting of a single forked bar in each branchial septum (b.s), and of two bars in each tongue-bar; g.p, gill-pore, opening on the dorsal surface of the trunk; g.s, gill-sac; s, synapticulum (only one or two shown); t, tongue-bar. The arrows indicate the communications of the gill-sacs with the exterior and with the pharynx.

In Amphioxus, as in most Enteropneusta, adjacent rods are connected at intervals by chitinous "synapticula" (Fig. 5, s), which traverse one or the other of the halves of the gill-slit. In Dolichoglossus, where no synapticula occur, the tongue-bars may be turned inside out by slight pressure, and then project to the exterior through the gill-pores.

The subdivision of the branchial region of the alimentary canal into two parts, as shown in Fig. 4, is characteristic of Glossobalanus and its allies. In Dolichoglossus and Glandiceps there is no such constriction, the region occupied by the gill-slits being merely the dorsal half of a tube with a simple circular section. Schizocardium (Fig. 6) agrees with Amphioxus in the fact that the gill-slits occupy nearly the whole of the wall of the pharynx; the only parts not perforated by gill-slits being the small dorsal and ventral portions.

In Ptychodera (Fig. 4), the gill-sacs are practically absent. The U-shaped slits of the pharyngeal wall thus open directly to the exterior,[^[20]] and can be seen from the outside. In species which have this arrangement, the genital wings are greatly developed, so as to arch over the back of the branchial region. The gill-slits thus open into a kind of "atrium," resembling that of Amphioxus in its relation to the gill-slits, and in having the generative organs on its outer side, but differing from it in being dorsal to the pharynx.

Fig. 6.—Schizocardium brasiliense; transverse section through the branchial region, showing the great extent of the branchial part (b) of the pharynx; the oesophageal part (o) is reduced to a mere groove; g, gill-pore; g.s, gill-sac; r, reproductive organ; s, synapticula (cf. Fig. 5); t, tongue-bar. The muscles of the body-wall are not indicated: in other respects the figure corresponds with Fig. 4, except for the absence of genital wings in this region of the body. (After Spengel.)

At a certain distance behind the branchial region, the alimentary canal in Balanoglossus and Schizocardium is produced into a series of outgrowths, into which food does not pass. These "liver-sacs" give rise to corresponding folds (Fig. 1, A, h) of the dorsal body-wall, a conspicuous external feature of the species in which they are present. The most interesting peculiarity of the digestive tract in this region is the existence, in certain species, of pores, possibly vestigial gill-slits, leading from it to the exterior.

Notochord and Skeleton.—The structure compared by Bateson with the Vertebrate notochord is a hollow dorsal outgrowth of the alimentary canal of the collar-region (Fig. 7, n). Near its origin it is slender, but in the proboscis it dilates into a comparatively large organ, which in most cases retains its cavity. Its cells have a vacuolated appearance, which recalls the fine structure of the Vertebrate notochord. In Schizocardium and Glandiceps, the organ is produced into a slender "vermiform process" (v), which extends nearly to the tip of the proboscis.

Fig. 7.—Schizocardium brasiliense; longitudinal, median section through the proboscis, the collar, and the first part of the trunk; b, main blood-space of the proboscis; b.c¹, b.c², b.c³, first, second and third body-cavities; c.m, circular muscles of proboscis; e, epidermis; l.m, longitudinal muscles of proboscis; m, mouth; n, notochord; n.s, central nervous system, continuous with the subepidermic nerve-plexus (black) of the proboscis, and with the dorsal nerve (d); p.c, pericardium; p.s, proboscis-stalk; s, proboscis-skeleton; v, vermiform process of notochord. (After Spengel.)

The main support of the proboscis-stalk is the "proboscis-skeleton" (s), a Y-shaped organ whose median part lies beneath the base of the notochord, its diverging legs extending backwards along the outer side of the alimentary canal of the collar. The proboscis-skeleton, like the branchial skeleton, is a special development of the structureless membrane which is found at the base of the layers of cells of Balanoglossus, and in most species it grows merely by the deposition of laminae of chitin from the notochord, and from the ventral epidermis of the proboscis-stalk.

In some species, however, and particularly in Balanoglossus aurantiacus and Glandiceps, the primary skeleton becomes surrounded by an extensive development of a secondary cartilaginoid skeleton, consisting of a structureless substance into which the adjacent body-cavities of the proboscis and collar send cellular outgrowths. The possibility of a relation between this tissue, more or less surrounding a part of the notochord, and the cartilage of Vertebrates cannot be overlooked.

The caudal region may be stiffened (?) by a "pygochord"[^[21]] which is a median derivative of the alimentary canal on its ventral side.

Vascular System and Proboscis-Gland.—The main vessels are a dorsal and a ventral vessel (Fig. 4, d.v, v), lying in their respective mesenteries. The details of the vascular system are complicated, and have not been thoroughly made out, the nearly colourless character of the blood making their investigation a difficult matter. The following points may, however, be noted. The blood is said to pass forwards in the dorsal vessel, which, like the ventral vessel and a pair of lateral vessels in the hepatic region, is contractile. In the collar the dorsal vessel lies between the two perihaemal spaces, on the dorsal side of the base of the notochord. The principal blood-space in the proboscis (Fig. 7, b) lies between the notochord (n) and an organ known as the "heart-vesicle" or "pericardium" (p.c). The latter has muscular walls and it contracts rhythmically in the larva. Its behaviour in the adult is not so easily made out, but it is probable that, although it does not communicate with the vascular system, its contractions propel the blood contained in the space immediately beneath it. The blood, after passing to a glandular organ, the "proboscis-gland" or "glomerulus," which lies at the sides and in front of the notochord, appears to pass round the collar to the ventral vessel. Various systems of vessels are connected with the skin, the gills, the alimentary canal and the generative organs.

The function of the proboscis-gland is possibly excretory. In this case it is probable that the proboscis-pore eliminates the waste products discharged by the gland into the anterior body-cavity, though this view is not favoured by Willey.

Reproductive Organs.—The sexes are separate, the reproductive organs consisting of a series of simple or branched glands which occur along the dorso-lateral lines of the anterior part of the body; being usually found throughout the branchial and generative regions and ending at the beginning of the hepatic region. The reproductive organs may pass into great extensions of the body-wall known as the "genital wings," specially developed in some species of Balanoglossus and Ptychodera (Figs. 1 A, 4).

Stereobalanus canadensis, a species with long slit-like external gill-pores, is interesting in possessing a well-developed genital wing both dorsally and ventrally to the series of gill-pores of each side.

Each reproductive gland opens by its own pore or pores directly to the exterior. Several glands and pores may occur in the same transverse section.

According to Spengel there is no definite relation between the number of the reproductive organs and that of either the gill-sacs or the liver-outgrowths. The only definite segmentation exhibited by Balanoglossus is thus the division into three regions which is so distinctly shown by the arrangement of the body-cavities; though the gill-sacs may indicate an incipient further segmentation of the major part of the body. In this connexion it is interesting to notice MacBride's statement[^[22]] that the body-cavity of Amphioxus develops in the embryo as five cavities, just as in Balanoglossus; the segmented part of the body being formed by a secondary segmentation of the third body-cavities.

Regeneration.—Balanoglossus, like Phoronis (p. [30]), possesses great powers of regenerating lost parts. The posterior part of the body is readily re-formed, while Spengel has shown[^[23]] that even the proboscis, collar and branchial region can be regenerated, apparently from a fragment of the body.

Genera of Enteropneusta.—Spengel, whose Monograph is indispensable to every student of the Enteropneusta, formerly proposed to divide the old genus Balanoglossus into four; but he now recognises no less than nine.[^[24]] Some of the more important characters are given below, but for the arrangement of the muscles, important from a systematic point of view, reference must be made to the original sources.

A. Notochord with a vermiform process (Fig. 7, v); pericardium with anterior diverticula more or less developed. .......... Glandicipitidae

(a) Liver-sacs and synapticula present; gill-slits almost equalling the pharynx in depth, so that the ventral, non-branchial part of the pharynx is reduced to a mere groove (Fig. 6); nerve-roots absent; pericardial diverticula long. .......... Schizocardium, Speng.

(b) Liver-sacs absent;[^[25]] ventral part of pharynx well developed; pericardial diverticula short.

(i.) Synapticula and nerve-roots absent. .......... Glandiceps, Speng.

(ii.) Synapticula present; nerve-roots present or absent; genital region with dermal pits. .......... Spengelia, Willey.

B. Notochord with no vermiform process; pericardium simple; ventral part of pharynx large, and sometimes more or less separated from the branchial part (Fig. 4).

(a) Liver-sacs,[^[26]] synapticula and nerve-roots present. .......... Ptychoderidae

(i.) Genital wings well developed.

(α) Gill-sacs opening by long slits. .......... Ptychodera, Eschsch.

(β) Gill-sacs opening by small pores. .......... Balanoglossus, Delle Chiaje.

(ii.) Genital wings hardly developed. .......... Glossobalanus, Speng.

(b) Liver-sacs, synapticula and nerve-roots absent. .......... Harrimaniidae

(i.) Proboscis long; one proboscis-pore. .......... Dolichoglossus, Speng.

(ii.) Proboscis short; two proboscis-pores.

(α) Two pairs of genital wings. .......... Stereobalanus canadensis, Speng.

(β) No genital wings. .......... Harrimania, Ritter.

The name Balanoglossus was introduced by Delle Chiaje in 1829 for B. clavigerus (Fig. 1, A), from the neighbourhood of Naples. As Spengel has shown, its etymology has been much misunderstood. The second half of the name refers to a fancied resemblance between the Balanoglossus, with its largely developed genital wings, and the tongue of an ox. Βάλανος means "acorn," and it has usually been supposed that this name was suggested by the resemblance of the proboscis, projecting from the collar, to an acorn in its cup, a view which finds its expression in the name "Eichelwurm" used by German zoologists. But the idea expressed by Delle Chiaje was really a similarity between the collar of Balanoglossus and the outer shell of Balanus, the barnacle or "acorn-shell" found everywhere on rocks between tide-marks.

Fig. 8.—Metamorphosis of Balanoglossus, probably of Balanoglossus biminiensis Willey, Bahama Islands. All the figures are magnified to the same scale (× 14). A, fully developed free-swimming larva, or Tornaria, side view; B, commencement of metamorphosis, side view; C, later stage, dorsal view. Increase in size takes place after this stage; a, anus; b.c¹, body-cavity of proboscis; c, collar; c.r, transverse ciliated ring; d.p (in A), dorsal pore (= proboscis-pore), seen also in C on the left side, just behind the reference line p.c; e, eyes and sensory thickening of skin (in A); g, gill-pore; g.s, gill-sacs, developing as outgrowths of the alimentary canal; three are already present in B, but are better seen in C, in which they are still without openings to the exterior; l, postoral part of the longitudinal band of cilia; l′, its praeoral part; both l and l′ are produced (in A) into tentacles, over which the band of cilia is looped; the groove in the middle of the figure, between l and l′, conducts the food by the transverse groove to the mouth (m); p.c, blood-space of proboscis and pericardium ("heart" of larva); s, stomach. (After Morgan).

Development.—The free-swimming, larval stage of Balanoglossus is known as Tornaria (Fig. 8, A). Several distinct forms of the larva are known,[^[27]] although it is not yet possible to refer them with certainty to their respective adults.

Tornaria was described and named by Johannes Müller, who regarded it as the larva of a Starfish,[^[28]] in spite of his intimate knowledge of the development of these animals. Its correct systematic position was first demonstrated by Metschnikoff in 1869.

The larva agrees with many other pelagic forms in being excessively transparent. The form described by Spengel as T. grenacheri attains the remarkable length of 9 mm. (nearly ⅖th inch).

The full-grown larva is usually ovoid, and a complicated "longitudinal" band of cilia runs in several loops over its anterior two-thirds. In side view, part of the surface limited by the ciliated band appears like a T with a double outline, the cross piece being bent downwards on each side, so as to form an anchor-like curve, the middle of which is at the anterior pole of the larva. In T. krohni, which occurs on our south coast,[^[29]] the ciliated band has a wavy course. In the West Indian larva[^[30]] shown in Fig. 8 A, the ciliated band is produced into numerous tentacles, which fringe the sides of the T-shaped areas or grooves of the surface. These grooves and the cilia which border them are used for conveying food to the mouth.[^[31]] At the apex of the larva is a thickening (e) of the ectoderm, bearing two eye-spots. The main locomotor organ is a simple transverse band (c.r) of "membranellae," vibratile structures composed of fused cilia. The mouth (m), on the ventral side, leads into the oesophagus, and this into the stomach (s). The latter is separated by a marked constriction from the intestine, which opens by the anus (a) at the posterior pole.

On the dorsal side is a pore, the "dorsal pore" (d.p.), which leads into a thin-walled sac (b.c¹) destined to become the proboscis-cavity of the adult. To the right of the dorsal pore lies the pulsating "heart," which apparently becomes the pericardium of the adult. Bourne and Spengel regard it as a right proboscis-cavity. In the older larvae, the second and third body-cavities appear as paired thin-walled sacs in close contact with the hinder part of the stomach. The skin is very thin, and the five body-cavities do not nearly fill the space between it and the alimentary canal. This space becomes obliterated for the most part by the enlargement of the body-cavities, and its last remains persist, as in many other animals,[^[32]] as the vascular spaces of the adult.

In Dolichoglossus kowalevskii, and probably in other species with large eggs,[^[33]] development proceeds by gradual stages to the adult form, and no Tornaria-stage is passed through. The opaque young animal, on being hatched, creeps about in the muddy sand in which the adult is found, later moving in a leech-like manner, by alternately attaching itself by its two ends. The young stages were ingeniously obtained by Bateson, to whom our knowledge of the development of this species is due,[^[34]] by allowing a large quantity of the mud to settle after being stirred up, the layer of the specific gravity corresponding with that of the young Balanoglossus being then separated by means of a siphon. The young stages previously contained in several hundredweight of mud were thus easily collected into a pint of water. Morgan recommends treating the layer obtained by a similar process with picric acid, which stains the young Balanoglossus yellow.

The embryo early becomes a "blastosphere" or hollow vesicle formed of a single layer of cells. One half of this is invaginated, or pushed into the other half, and a "gastrula" is thus developed, the cavity of which is the "archenteron," and the two cell-layers respectively "ectoderm" and "endoderm." The "blastopore," or orifice of invagination, is at the posterior pole of the larva, where it narrows and closes, the locomotor, transverse band of cilia developing round it. No other bands of cilia appear in this form of development. The proboscis becomes marked out externally by the appearance of a circular groove, near the middle; and behind this groove a second one appears, which forms the posterior boundary of the collar. The larva, which now resembles Fig. 8 C, is usually hatched at this stage. Two gill-slits make their appearance, and the mouth and anus are perforated; the anus being in the position of the blastopore.

The body-cavities are formed as five derivatives of the archenteron. One of these is unpaired, and becomes the proboscis-cavity; while the others are the paired cavities of the collar and trunk (cf. Fig. 2). There is some uncertainty about the origin of the body-cavities of the free-swimming Tornaria, although it seems most probable that they are developed either from the wall of the stomach or intestine,[^[35]] or from scattered mesoderm cells[^[36]] which lie in the segmentation-cavity.

The metamorphosis of Tornaria is accompanied by a great diminution in size,[^[37]] probably due to the loss of water; by this cause and by the simultaneous thickening of the skin, the larva loses its transparency.

The external features of the metamorphosis are sufficiently indicated by Fig. 8, the ciliated bands finally disappearing. The dorsal pore persists as the proboscis-pore; the notochord and numerous gill-slits are developed as outgrowths of the alimentary canal, the reproductive organs make their appearance, probably from the mesoderm,[^[38]] the trunk meanwhile elongating so that the proportions of the adult are acquired.

Order II. Pterobranchia.

Tubicolous Hemichordata, with one pair of gill-slits or none, a U-shaped alimentary canal, and a dorsal anus situated near the mouth. Proboscis flattened ventrally into a large "buccal disc," its base covered dorsally by the collar, which is produced into two or more tentaculiferous arms. Trunk short, prolonged into a stalk. Reproduction by budding occurs.

This group consists of the two genera Cephalodiscus (Fig. 9) and Rhabdopleura (Fig 12). The latter, first dredged by G.O. Sars, in 1866, from 120 fathoms off the Lofoten Islands, was included in a catalogue of deep-sea animals published by his father, M. Sars, in 1868 as Halilophus mirabilis, a name which has been superseded by Rhabdopleura normani, Allman,[^[39]] based on specimens dredged by Canon Norman in 90 fathoms, off the Shetland Islands.

Fig. 9.—Cephalodiscus dodecalophus, M‘Intosh, Straits of Magellan; A, small portion of the common "house," × 1; a, a single individual, shown also as B, × 65; six of the tentacular arms, belonging to the collar, are seen springing from behind the proboscis or "buccal disc." This has a crescentic band of pigment parallel with its posterior border, which conceals the mouth. The stalk, bearing a bud, which already shows the beginning of two tentacular arms, is seen to the right. (After M‘Intosh, B from Parker and Haswell.)

The structure of Rhabdopleura has been described by Sars,[^[40]] Lankester,[^[41]] and Fowler.[^[42]] R. normani is common in certain Norwegian Fjords, at depths of 40 fathoms or more, and has been recorded by Fowler from the Tristan d'Acunha group in the S. Atlantic; R. compacta has been found off the N.E. coast of Ireland[^[43]] and near Roscoff, on the N. coast of Brittany; while forms described by Jullien[^[44]] as R. grimaldii and R. manubialis have been dredged off the Azores. I have recently found a fragment of Rhabdopleura from South Australia. It is doubtful how far these species are distinct.

Cephalodiscus dodecalophus[^[45]] was found in the Straits of Magellan, during the "Challenger" voyage, at a depth of 245 fathoms, and has recently been rediscovered in shallower water in the same neighbourhood by the Swedish Antarctic Expedition. Another Cephalodiscus, at present undescribed, has been obtained by Dr. Levinsen from 100 fathoms off the coast of Japan; while the Dutch expedition carried out by the "Siboga" has resulted in the discovery of two other specimens, one from a reef close to low-tide mark on the coast of Borneo, the other from 41-52 fathoms off Celebes. These three specimens differ markedly from one another and from the "Challenger" specimen of C. dodecalophus, and it is probable that they all belong to new species. The occurrence of a deep-sea animal at a great distance from the locality at which it was first found is not in itself a matter for great surprise; but in the present instance two of the newly discovered forms are from shallow water, and one of them is actually littoral. The occurrence of so many species of Cephalodiscus in Oriental waters suggests that the Pacific or the Indian Ocean may be the headquarters of the genus, which may prove to be far less of a rarity than has hitherto been supposed. There is evidence derived from the results of the "Siboga" expedition that abyssal animals may migrate into comparatively shallow water in the Malay Archipelago.

Cephalodiscus and Rhabdopleura are remarkable for their power of producing buds. In the former these arise from the apex of a stalk which is given off on the ventral side of the body, and they break off when they reach a certain age; in the latter they do not become free, and a colony results, which consists of a creeping "stolon" from which vertical branches are given off at intervals, each ending in an individual of the colony. Cephalodiscus forms a gelatinous "house" (Fig. 9, A), in the passages of which are found large numbers of the free individuals, together with their eggs and embryos. Rhabdopleura (Fig. 12) is protected by cylindrical tubes, one of which corresponds with each individual.

Fig. 10.—Longitudinal median section of Cephalodiscus dodecalophus. a, Anus; b.c¹, b.c², b.c³, first, second, and third body-cavities; int, intestine; m, mouth; nch, notochord; n.s, central nervous system; oes, oesophagus; op, operculum, the ventro-lateral part of the collar; ov, ovary; ovd, pigmented oviduct; ph, pharynx; p.p, proboscis-pore; ps, proboscis; st, stomach; stk, stalk.

Cephalodiscus, though no more than two or three millimetres in length, is provided with practically all the important organs possessed by Balanoglossus. Its proboscis or "buccal shield" (Fig. 10, ps) is a large flattened structure, which overhangs and entirely conceals the mouth. The anterior body-cavity opens to the exterior by two symmetrically placed proboscis-pores (p.p), just in front of the tip of the notochord (nch). The collar, which has paired body-cavities, is produced dorsally into 4-6 pairs of plume-like arms, which bear an immense number of pinnately-arranged tentacles. The arms, which may end in a swollen bulb,[^[46]] have ventral grooves along which food doubtless travels to the mouth by ciliary currents. The anterior edge of the ventral half of the collar is drawn out into a narrow flap or operculum (Fig. 11, op), in front of which is the mouth, and behind it the gill-slits (g) and collar-pores (c). The central nervous system (n.s) is a thick mass of nerve-tissue in the dorsal epidermis of the collar; it is not sunk beneath the skin as in Balanoglossus. The details of the nervous and vascular systems, and the development of the buds, have been described by Masterman. In the dorsal region of the collar the alimentary canal has a slender diverticulum, the notochord, which passes into the base of the proboscis; it is believed by Masterman to have a function similar to that of the neural gland (cf. p. [52]) of Tunicates.

The next part of the alimentary canal, the pharynx,[^[47]] has a single pair of simple gill-slits opening to the exterior immediately behind the collar-pores. The short oesophagus (Fig. 10, oes) is followed by the wide stomach (st), and this by the intestine (int), which opens by the anus (a) near the front end of the body.

Fig. 11.—Longitudinal section through Cephalodiscus dodecalophus, passing through the two sides of the body; a, tentacular arm; b.c², collar-cavity; b.c³, trunk-cavity; c, collar-pore; g, gill-slit; i, intestine; n.s, central nervous system; o, oesophagus; op, operculum; p, pharynx; s, stomach.

The trunk contains paired third body-cavities (b.c³), the septum between which and the collar-cavities is slightly behind the line of origin of the operculum. Two ovaries (ov) are situated between the pharynx and the last part of the intestine, each opening to the exterior dorsally between the central nervous system and the anus. Each oviduct (ovd) contains dark pigment, which is seen through the dorsal skin on removing the tentacular arms. Eggs, each enclosed in a stalked membrane, occur in numbers in the cavities of the gelatinous house. The early stages of the development are passed through inside the tubes; but there is at present little other information with regard to the embryonic development of the Pterobranchia. The specimen obtained by the "Siboga" from Celebes is a male colony with dimorphic individuals, the reproductive organs being confined to two-armed zooids with vestigial alimentary canal.

Fig. 12.—Small portion of colony of Rhabdopleura normani, Allman, Lofoten Islands, × 16. a, Anus; p, proboscis (= buccal disc); r, rod-like axis of the adherent part of the colony, prolonged into s, the stalks of the individuals; st, stomach; t, the two tentacular arms of the collar. (After Sars.)

Rhabdopleura differs from Cephalodiscus in its much smaller size,[^[48]] and it is perhaps due to its minuteness that it does not possess certain organs found in the latter. The stalk is represented by a long muscular cord, which is merely a narrow part of the body. Basally the stalk of each individual passes into a common axis, which is for the most part attached to the substance on which the colony is growing, and is to some extent branched. The muscular stalk can be contracted into a spiral, thereby retracting the animal into its tube. The stalks and the younger parts of the axis which connects them are soft, but the older parts secrete a dark brown cuticle, forming a narrow tube which becomes embedded in the adherent wall of the outer tube. The thin dark axis, to which the name Rhabdopleura refers, is the feature by which the animal can most readily be recognised without magnification.

The outer transparent tube is constructed by the proboscis, or buccal shield, the secretion of which appears to be intermittent, so that the tube consists of a series of rings piled on one another. The animal crawls up the inside of its tube by means of its proboscis, while it is retracted by means of the muscles of its stalk.

The growing axis ends in a row of young buds, the buccal shields of which early reach a relatively large size. The terminal bud gives rise to tube-rings, so that the axis is surrounded by a cylindrical outer tube, which becomes interrupted by transverse septa, each bud, except the end one, thus lying in a closed chamber. The wall of each chamber becomes perforated, and the buccal shield then prolongs this perforation by adding tube-rings, the formation of which continues till the tube reaches a considerable length. The bud remains connected with the axis by means of its narrow proximal region, which forms its stalk. The adherent part of the adult colony thus consists of a row of short tubes, traversed by the common axis of the colony. Each tube is produced laterally into the upright tube of an individual.

The general anatomy closely resembles that of Cephalodiscus.[^[49]] There are five body-cavities and a notochord. Collar-pores exist, but proboscis-pores and gill-slits have not been described. The dorsal region of the collar bears only a single pair of arms.

Order III. Phoronidea.

The structure and development of Phoronis (Fig. 13), have already been described in Vol. II.[^[50]] of this series; and Masterman's investigations, then published in a preliminary form only, are there alluded to. Since then this author has published fuller accounts[^[51]] of his results, which, if substantiated, would indicate a near relationship between Cephalodiscus and Phoronis.

Phoronis is a small tubicolous animal, of gregarious habits, which has usually been regarded as related to the Gephyrea. Its body ends in a plume of ciliated tentacles, which can be protruded from its tube, and the anus is on the dorsal side, not far from the mouth. In both these respects it agrees with Cephalodiscus, but a more striking similarity is asserted by Masterman to exist between the latter and Actinotrocha, the larval stage of Phoronis. The prae-oral ciliated hood (Fig. 14) of Actinotrocha is regarded as the proboscis, and it contains a median cavity, traversed, like that of Balanoglossus, by muscular fibres. The collar is the region between the constricted neck and an oblique line, parallel to and immediately behind the series of tentacles, which thus belong to the collar. This division has a collar-cavity which is said to be distinct from the prae-oral cavity, and is separated by a septum from the posterior body-cavity. Its dorsal epidermis contains the central nervous system (n.s), which is connected with a system of nerves resembling those of Balanoglossus. A median diverticulum of the alimentary canal of this part may be compared with the notochord of that animal, but there are no gill-slits.

Fig. 13.—Phoronis buskii, M‘Intosh, Philippine Islands, x about 2. (After M‘Intosh, from Shipley.)

The remainder of the body of Actinotrocha corresponds with the trunk of Balanoglossus. Its body-cavity is distinct from that of the collar, and is divided by a ventral mesentery, though not by a dorsal mesentery. A noteworthy fact is that both Actinotrocha and Tornaria swim by means of a ring of strong cilia or membranellae[^[52]] which surrounds the anus.

Fig. 14.—Actinotrocha-larva of Phoronis. a, Anus; b.c¹, b.c², b.c³, first, second and third body-cavities; c, circular nerve, running in the posterior boundary of the collar, immediately behind the ring of tentacles; c.r, ciliated ring; d, diverticulum (paired) of alimentary canal; m, mouth; n.s, central nervous system; p, nerve running round the ventral border of the proboscis; s, sense-organ; s.s, subneural sinus, a vascular space whose hind wall is constituted by the front boundary of b.c², its front wall being formed by the hind wall of b.c¹; in this region is seen a median outgrowth of the alimentary canal, which may be compared with the notochord of Cephalodiscus, or of the young Tornaria (cf. Morgan, J. Morphol. v. 1891, Plate xxvi. Fig. 40.) (After Masterman.)

Important memoirs on the structure of Actinotrocha have recently been published by Ikeda,[^[53]] de Selys Longchamps,[^[54]] Goodrich,[^[55]] and Schultz,[^[56]] who criticise many of Masterman's statements. While it is admitted on all sides that an oblique septum following the line of the bases of the tentacles completely subdivides the body-cavity, Masterman's account of the anterior cavities is not confirmed, the spaces indicated by b.c¹ and b.c² in Fig. 14 being stated to be really continuous with one another, while the "subneural sinus" (s.s) is regarded as a part of this space. It appears, however, from the account given by Ikeda, and followed by Goodrich, that the old Actinotrocha has two distinct spaces in front of the septum. The first of these corresponds with b.c¹ + most of b.c² in Fig. 14, and is continuous with the cavities of the larval tentacles. Into it project the blind ends of the larval excretory organs, which, according to Goodrich, bear numerous "solenocytes" similar to those described by the same author in Amphioxus and in Polychaet worms (Fig. 79, p. [127]). The second cavity is a relatively small crescent (not shown in Fig. 14), lying on the anterior face of the septum, the tips of the crescent nearly meeting dorsally, so as to constitute an almost complete ring following the bases of the tentacles, into each of which it gives off a blind outgrowth. At the metamorphosis, the crescentic space becomes the prae-septal body-cavity and the cavities of the tentacles of the adult, the circular blood-vessel of which is formed from the remains of the large prae-septal space of the larva. Schultz, in calling attention to the fact that both Phoronis and its larva have a striking power of regenerating lost parts, confirms the conclusion that this animal belongs to the Hemichordata. He gives reasons, however, for believing that it is in the adult Phoronis rather than in the larval Actinotrocha that it is possible to discover the most satisfactory evidence of this affinity.

The metamorphosis[^[57]] of Actinotrocha is very remarkable, and is accompanied by the eversion of a ventral ingrowth of the body-wall. A loop of the alimentary canal passes into this eversion, which becomes the main part of the body of the adult; and the anus is thereby brought relatively nearer the mouth than in the larva. The occurrence of this process may help to explain the position of the anus in the Pterobranchia.

Affinities of the Hemichordata.—There can be no doubt that some of the resemblances, in structure and in development, between Balanoglossus and certain Vertebrates are extremely striking. The view that Balanoglossus is related to the ancestors of Vertebrates[^[58]] appears to exclude other views[^[59]] which have been suggested with regard to the same question. The Balanoglossus-theory does not explain the similarity between the segmentation and the excretory systems of Vertebrates and Chaetopods; but, on the contrary, there are important characters which Vertebrates share with Balanoglossus but with no other "Invertebrates." Of these the most important appear to be the resemblances between the gill-slits and gill-bars of Balanoglossus and Amphioxus; the position, structure and mode of development of the central nervous system; and the presence of a structure in the Hemichordata, which may be regarded as the notochord. There are other points in which Balanoglossus specially resembles Amphioxus, such as the early development, the mode of formation of the body-cavities,[^[60]] and the presence of numerous generative organs.

All these, taken together, make it necessary to consider carefully the claims of Balanoglossus to relationship with the ancestors of Vertebrates in making any speculations on this interesting problem.

However improbable it may appear at first sight, it is possible to hold the view that Balanoglossus is related at the same time to Vertebrates and to Starfishes and other Echinoderms. The similarity between a young Tornaria and a young Bipinnaria-larva of a Starfish is so great as to have misled even Johannes Müller. The more obvious resemblances are the almost identical course of the longitudinal ciliated band in the young stages, and the presence of a dorsal pore. The Echinoderm-larva is not, however, provided with eye-spots, nor has it the posterior, or transverse, ciliated band of Tornaria.

Recent studies on the development of Echinoderms[^[61]] have made it probable that the five body-cavities of Balanoglossus are represented in the larvae of those animals; and this materially strengthens the probability of the view that the respective adults are also allied.[^[62]] It may be added that the relationship which appears to be indicated is between Balanoglossus and the bilateral ancestors from which the radially-symmetrical Echinoderms are probably descended.

In comparing the Enteropneusta with the Pterobranchia, the disproportionate size of the trunk of Balanoglossus may perhaps be explained by assuming that the region of the third body-cavities has been enlarged since Balanoglossus branched off from the ancestral stock.[^[63]] The approximation of the anus to the mouth in Pterobranchia is perhaps the result of their tubicolous habits.[^[64]] In the position of the central nervous system in the skin of the collar, Cephalodiscus appears to be more primitive than Balanoglossus, as has been pointed out by Morgan.[^[65]] It is not impossible that the presence of one pair of gill-slits in Cephalodiscus indicates that this animal diverged from the ancestors of Balanoglossus before the gill-slits were metamerically repeated.

ASCIDIANS AND AMPHIOXUS

BY

W. A. HERDMAN, D.Sc. (Edinb.), F.R.S.

Professor of Natural History in the University of Liverpool

CHAPTER II

TUNICATA (ASCIDIANS AND THEIR ALLIES)

INTRODUCTION—OUTLINE OF HISTORY—STRUCTURE OF A TYPICAL ASCIDIAN—EMBRYOLOGY AND LIFE-HISTORY

The Tunicata are marine animals found in practically all parts of the sea, and at all depths. They extend from the Arctic and Antarctic regions to the tropical waters, and from the littoral zone down to the abyssal depths of over three miles. They are abundant in British seas. They vary greatly in shape and colour, and range in size from an almost invisible hundredth of an inch to large masses a foot or more in diameter. And yet most Tunicata have a characteristic appearance by which they can be readily distinguished from other animals. They form a well-defined group, with definite anatomical characters, and there are no known forms intermediate between them and other groups. The Tunicata were formerly regarded as constituting, along with the Polyzoa and the Brachiopoda, the Invertebrate Class "Molluscoidea." They are now known to be a degenerate branch of the lower Chordata, and to be more nearly related to the Vertebrata than to any group of Invertebrates.

Tunicata occur either fixed or free, solitary, aggregated or in colonies (see Fig. 27, p. [64]). The fixed forms, found on the sea-bottom, are usually termed "Ascidians," those that are solitary or merely aggregated being "Simple Ascidians" or Monascidiae, and those that are organically united into a colony being "Compound Ascidians" or Synascidiae. The colonies have been produced by budding, a process which is very general in the group, and the members of the colony are conveniently known as "Ascidiozooids." Some exhibit alternation of generations, and all pass through remarkable changes in their life-history, nearly all of them undergoing a retrogressive metamorphosis.

Outline of History.

More than two thousand years ago Aristotle gave a short account of a Simple Ascidian under the name of Tethyum. He described the appearance and some of the more important points in the anatomy of the animal. From that time onwards comparatively little advance was made until Schlosser and Ellis, in a paper on Botryllus, published in the Philosophical Transactions of the Royal Society for 1756, first brought the Compound Ascidians into notice. It was not, however, until the commencement of the nineteenth century, as a result of the careful anatomical investigations of Cuvier[^[66]] upon the Simple Ascidians, and of Savigny[^[67]] upon the Compound Ascidians, that the relationship between these two groups of Tunicata was conclusively demonstrated. Up to 1816, the date of publication of Savigny's great work, the few Compound Ascidians previously known had been generally regarded as Alcyonaria or as Sponges; and although many new Simple Ascidians had been described by O. F. Müller[^[68]] and others, their internal structure had not been investigated. Lamarck[^[69]] in 1816, chiefly as the result of the anatomical discoveries of Savigny and Cuvier, instituted the class Tunicata, which he placed between the Radiata and the Vermes in his system of classification. The Tunicata included at that time, besides the Simple and the Compound Ascidians, the pelagic forms Pyrosoma, which had been first made known by Péron in 1804, and Salpa described by Forskål in 1775.

Chamisso, in 1819, made the important discovery that Salpa in its life-history passes through the series of changes which were afterwards more fully described by Steenstrup in 1842 as "alternation of generations"; and a few years later Kuhl and Van Hasselt's investigations upon the same animal resulted in the discovery of the alternation in the directions in which the wave of contraction passes along the heart, and in which the blood circulates through the body. It has since been found that this observation holds good for all groups of the Tunicata. In 1826, H. Milne-Edwards[^[70]] and Audouin made a series of observations on living Compound Ascidians, and amongst other discoveries they found the free-swimming tailed larva and traced its development into the young Ascidian.

In 1845, Carl Schmidt[^[71]] first announced the presence in the test of some Ascidians of "tunicine," a substance very similar to cellulose; and in the following year Löwig and Kölliker[^[72]] confirmed the discovery, and made some additional observations upon this substance and upon the structure of the test in general. Huxley,[^[73]] in an important series of papers published in the Transactions of the Royal and Linnean Societies of London from 1851 onwards, discussed the structure, embryology, and affinities of the pelagic Tunicates, Pyrosoma, Salpa, Doliolum and Appendicularia. These important forms were also investigated about the same time by Gegenbaur, Vogt, H. Müller, Krohn, and Leuckart.

The most important epoch in the history of the Tunicata is the date of the publication of Kowalevsky's celebrated memoir[^[74]] upon the development of a Simple Ascidian. The tailed larva had been previously discovered and investigated by several naturalists, notably by H. Milne-Edwards,[^[75]] P. J. van Beneden, and Krohn; but its minute structure had not been sufficiently examined, and the meaning of what was known of it had not been understood. It was reserved for Kowalevsky in 1866 to demonstrate the striking similarity in structure and in development between the larval Ascidian and the Vertebrate embryo. He showed that the relations between the nervous system, the notochord, and the alimentary canal are practically the same in the two forms, and have been brought about by a very similar course of embryonic development. This discovery clearly indicated that the Tunicata are closely allied to Amphioxus and the Vertebrata, and that the tailed larva represents the primitive or ancestral form from which the adult Ascidian has been evolved by degeneration. This led naturally to the view usually accepted at the present day, that the group is a degenerate side-branch from the lower end of the phylum Chordata, which includes the Tunicata (or Urochordata), Balanoglossus and its allies (Hemichordata), Amphioxus (Cephalochordata), and the Vertebrata (or Craniata). Kowalevsky's great discovery has since been confirmed and extended to all other groups of the Tunicata by Kupffer,[^[76]] Giard, and others.

In 1872 Fol[^[77]] added largely to the knowledge of the Appendiculariidae, and Giard[^[78]] to that of the Compound Ascidians. The latter author described a number of new forms and remodelled the classification of the group. The most important additions which have been made to the Compound Ascidians since Giard's work have been the species described by von Drasche,[^[79]] from the Adriatic, and those discovered by the "Challenger" expedition.[^[80]] The structure and the systematic arrangement of the Simple Ascidians have been discussed of recent years mainly by Alder[^[81]] and Hancock, Heller,[^[82]] Lacaze-Duthiers,[^[83]] Traustedt,[^[84]] Roule, Hartmeyer, Sluiter[^[85]] and Herdman.[^[86]] In 1874 Ussoff investigated the minute structure of the nervous system and of the underlying gland, which was first discovered by Hancock, and showed that the gland has a duct which communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or "olfactory") tubercle. In an important paper published in 1880, Julin[^[87]] drew attention to the similarity in structure and relations between this gland and the "hypophysis cerebri" of the Vertebrate brain, and insisted upon their homology. Metcalf has recently added further to our knowledge on this and related matters.

The Thaliacea or pelagic Tunicata have of late years been the subject of several very important memoirs. The researches of Todaro, Brooks,[^[88]] Salensky,[^[89]] Seeliger,[^[90]] Korotneff,[^[91]] and others have elucidated the embryology, the gemmation and the life-history of the Salpidae; and Grobben, Barrois,[^[92]] and more especially Uljanin,[^[93]] have elaborately worked out the structure and the details of the complicated life-history of the Doliolidae. Finally we owe to the labours of Metschnikoff, Kowalevsky, Giard, Hjort, Seeliger, Ritter, Van Beneden and Julin, much detailed information as to development and life-history, the process of gemmation and the formation of colonies, which has added greatly to our knowledge of the position and affinities of the Tunicata and of their natural classification.

Structure of a Typical Ascidian.

If a typical "Simple Ascidian," such as the common British Ascidia mentula (Fig. 15), or Ascidia virginea, be examined alive and expanded in sea-water it will be seen to bear on the upper surface two short projections, each terminated by a wide tubular opening, through which the animal, when touched, can emit jets of water with considerable force—thus accounting for the popular name "sea-squirts." The rest of the body is covered by the dull grey tough cuticular outer "test" or "tunic" (hence Tunicata) by means of which the animal is attached to a rock or other foreign body. One of the tubular openings, the mouth or "branchial aperture," is terminal, and indicates the morphological anterior end; it is surrounded by eight lobes. The other opening, the cloaca or "atrial aperture," is on the dorsal edge, from one-third to one-half way down the body, and is bounded by six lobes only; consequently the two apertures, and so the ends of the body, can be distinguished externally by the number of lobes—an important matter. The area of attachment is usually the posterior part of the left side; in Fig. 15 the animal is seen from the right hand side.

If a little carmine-powder, or some other insoluble particles be scattered in the water in which the Ascidian is living, the particles will be seen to converge to the branchial aperture and be sucked in by the inhalent current entering the body. After a short interval a certain proportion of the particles will be shot out from the atrial aperture with the exhalent current.

These particles have passed through the pharyngeal portion of the alimentary canal and the cloacal passages, with the water used in respiration, but a considerable amount of such particles taken in with the water do not reappear, as they are retained by the nutritive organs and pass along the remainder of the alimentary canal with the food. The current of water passing in at the branchial and out at the atrial aperture is of primary importance in the life of the Ascidian. Besides serving for respiratory purposes it conveys all the food into the body and removes waste matters both intestinal and renal, and also expels the reproductive products from the body.

Fig. 15.—Ascidia mentula Linn. from the right side (natural size), Loch Fyne, N.B.; Br, Branchial aperture; At, atrial aperture. Arrows show the direction of the water currents.

The Test.—The test is notable amongst animal structures for containing "tunicine," a substance which appears to be identical in composition, and in behaviour under treatment with various reagents, with cellulose. It is cartilaginous in appearance and consistency, and to some extent in structure, as it consists of a clear (or in some cases fibrillated) matrix in which are embedded many corpuscles or cells. It is the matrix that contains the cellulose, which may form over sixty per cent by weight of the entire test. As the test is morphologically a cuticle, being a secretion on the outer surface of the ectoderm (Fig. 16, ec), the cells it contains have immigrated to it from the body, and it has recently been shown that many of these are mesodermal cells (leucocytes or connective tissue wandering cells, amoebocytes, and in some cases embryonic "kalymmocytes," or egg-follicle cells, see below, p. [56]), which have passed through the ectoderm. This process commences in the larval state with the migration of mesenchyme cells from the blastocoele through the epiblast. Ectoderm cells, and possibly also some primitive endoderm cells, also take part in forming the test. Many of these cells in the test remain small and simple, as the fusiform and stellate test-cells; some become pigment-cells, while others enlarge and become vacuolated to form the large (up to 0.15 mm. in diameter) vesicular or "bladder" cells—this is especially the case in the outer layer of the test in Ascidia mentula (see Fig. 17, bl) where there are innumerable clear vesicles, each surrounded by a thin film of protoplasm and having the nucleus still visible at one point of the surface. In some of the Tunicata the test-cells produce calcareous spicules of various shapes (see below, p. p. [86]).

Fig. 16.—Diagrammatic section through test and mantle of Ascidia to show the relations of ectoderm to body-wall and cuticle. bl.c, Bladder-cells; bl.s, blood-sinus; c.t.c, connective tissue cells; ec, ectoderm; mes.c, wandering mesoblast cells; m.f, muscle fibres; t.c, test-cells; t.v, "vessel" of the test."

The test also becomes organised by the growth into it of the so-called "vessels." These are outgrowths of the body-wall covered by ectoderm and containing prolongations of blood-channels from the connective tissue of the "mantle" (body-wall). Fig. 16, t.v shows such an outgrowth, and exhibits the general relations of test (cuticle), ectoderm, and mesoderm. It also explains how it is that the blood-channel being pushed out as a loop gives rise to the double or paired "vessels" seen branching through the test (see Fig. 17, v). The two vessels of a pair are one blood-channel imperfectly divided by a connective-tissue septum. The blood courses out along one side, round the communication in a "terminal knob" at the end, and back down the other side. The "terminal knobs" are very numerous, and form a marked feature in the outer layer of the test (Fig. 17, t.k); in some cases (Culeolus murrayi), they probably form an accessory organ of respiration, while in others (Botryllidae), they pulsate and aid in keeping up the circulation.

The ectoderm is a simple epithelial layer (Fig. 16, ec). It is turned in for a short distance at the branchial aperture (mouth), and atrial aperture (cloaca), as a short stomodaeum and proctodaeum respectively, lined in each case by a delicate prolongation of the test.

Fig. 17.—Section through the surface layer of test of Ascidia mentula, × 50. bl, Bladder cells; t.c, test cell; t.k, terminal knobs of vessels; v, vessels of test.

Fig. 24, A, p. [52], shows the relations of ectoderm, mesoderm, and endoderm in a section through the antero-dorsal part of the body. The cavity marked p.br is a portion of the atrial cavity lined by ectoderm, and must not be confounded with a coelom. The absence of a true coelom in the mesoderm will be noticed in this and other figures, and yet the Tunicata are Coelomata, although it is very doubtful whether the enterocoel which has been described in the development of some is ever found. The coelom is in any case largely suppressed later, and is only represented in the adult by the pericardium and by small cavities in the renal and reproductive organs and ducts.

Body-Wall and Cavities of the Body.—The name "mantle" is given to the ectoderm with the parietal mesoderm which form the body-wall inside the test. It is largely formed of connective tissues—both homogeneous and fibrous—with cells, blood-sinuses, and many muscle-bundles large and small running circularly, longitudinally, and obliquely, and interlacing in all directions (Fig. 18, m). The muscles are all formed of very long fusiform non-striped fibres. The mantle in some Ascidians is often brilliantly pigmented—red, yellow and opaque white, the coloured cells being exactly like those found in the blood.

Fig. 18.—Dissection of Ascidia, from right side, to show anatomy. a, Anus; At, atrial aperture; Br, branchial aperture; br.s, br.s′, branchial sac; end, endostyle; g.d, genital ducts; gon, ovary; hyp, neural gland; hyp.d, the duct leading to dorsal tubercle; m, mantle; n.g, ganglion; oes, oesophagus; p.br.c, peribranchial cavity; ren, renal vesicles; st, stomach; t, test; tn, tentacles; ty, typhlosole.

The mantle forms two well-marked siphons or short wide tubes, which lead in from the branchial and atrial apertures. These are surrounded by strong sphincter muscles,[^[94]] and are lined by the invaginated ectoderm and test. The one leads into the branchial sac or modified pharynx, and the other into the atrial or peribranchial cavity (see Fig. 18, and Fig. 19, p.br).

Figs. 18 and 19 show the relations of the branchial and peribranchial cavities to one another. The peribranchial cavity opens to the exterior dorsally by the atrial aperture, forms the cloaca along the dorsal edge of the body, and has extensions laterally on each side of the branchial sac, with the interior of which it is placed in communication by the secondary gill-slits or "stigmata" (Fig 19, sg). Along the ventral edge the mantle is united to the wall of the branchial sac, and it is only this union (Fig. 19, end) that prevents the peribranchial cavity from completely surrounding the branchial sac.

The following list of the cavities present in the body of the adult Ascidia may be useful at this point:—

1. The alimentary canal, including the branchial sac. This is derived from the archenteron of the embryo, is lined throughout by endoderm, and the system of cavities of the intestinal gland is to be regarded merely as an outgrowth from the alimentary canal.

2. The peribranchial (atrial) cavity, derived from two lateral ectodermal invaginations which join dorsally to form the cloaca and open to the exterior by the atrial aperture.

3. The original embryonic segmentation cavity (blastocoele) remains, where not obliterated by the development of the mesodermal connective tissue, as the irregular system of blood spaces, with its outgrowths in test and branchial sac. The heart, which has differentiated muscular walls, becomes secondarily connected at its ends with these blood spaces.

4. The pericardium and epicardium (see p. [83]) originate as outgrowths from the archenteron. They may therefore be regarded as enterocoelic spaces. The pericardium becomes completely closed off and separated from the alimentary canal. The epicardium may form paired tubes of great length, and may remain permanently connected with the branchial sac.

5. The cavities of the renal vesicles and of the gonads and ducts are spaces formed in the mesoblast. They have been variously interpreted:—

(a) As of the same nature as the blood spaces (blastocoelic), or

(b) As formed by a splitting of the mesoblast (coelomic).

6. The cavity of the neural gland and of its duct opening at the dorsal tubercle is derived from the primitive dorsal neural tube of the embryo, and so may be regarded as a part of the lumen of the cerebro-spinal nervous system.

Tentacles, etc.—The branchial aperture leads through the branchial siphon into the branchial sac. At the base of the siphon, just about the line of junction of the ectoderm of the stomodaeum with the endoderm of the mesenteron, is placed a circle of simple hair-like tentacles (Fig. 18, tn) which stand out at right angles to the wall, and more or less completely meet in the centre to form a delicate, sensory grid or sieve through which all the water entering the body has to pass. These tentacles not only act mechanically, but are also sensitive although only scattered sensory cells, and no specially differentiated sense-organs are found upon them. Behind the tentacles lies the plain, or papillated, prebranchial zone (Fig. 21, p.br.z), bounded behind by a pair of parallel and closely placed ciliated ridges with a groove between—the peripharyngeal bands—which encircle the anterior end of the branchial sac.

Fig. 19.—Semi-diagrammatic transverse section of Ascidia, passing through the atrial aperture, seen from anterior surface, left side uppermost. At, Atrial aperture; at.l, atrial lobe; Br.s, branchial sac; cl, cloaca; con, connective; d.bl.s, dorsal blood-sinus; d.l, dorsal lamina; end, endostyle; g.d, genital ducts; i, i′, intestine; l.v, interstigmatic vessel; m, mantle; m.b, muscle-bundles; ov, ovary; p.br, peribranchial cavity; r, rectum; ren, renal vesicles; sg, stigmata; sph, atrial sphincter; t, test; tr, transverse vessel; ty, typhlosole; v.bl.s, ventral blood-sinus.

The branchial sac is very large—much the largest organ of the body—and extends almost to the posterior end of the body, while the rest of the alimentary canal lies upon its left side. The food particles, consisting of microscopic plants and animals, are carried in through the branchial aperture by the current of water, but most of them do not pass out through the gill-slits to the atrium, being entangled in the viscid mucus which passes by ciliary action along the groove between the peripharyngeal bands.

Endostyle.—The mucus just referred to is produced in the long canal-shaped gland called the endostyle or hypobranchial groove, which runs along the entire ventral edge of the branchial sac (Fig. 18, end). The sides, and especially the floor of the endostyle, are richly ciliated, while there are four (or six) strongly-marked, peculiarly-shaped glandular tracts, two (or three) on each side (Fig. 20, gl) running along its length, and separated by areas of closely-packed fusiform cells with short cilia, amongst which are found some bipolar sensory cells.

Fig. 20.—Transverse section of the endostyle of Ascidia mentula, × 350. bl.s, Blood-sinus; end.l, lips of the endostyle; gl, glandular tracts; i.l, internal longitudinal bar; l.v, interstigmatic vessels; m, mantle; p.br, peribranchial cavity; sg, stigmata; v.bl.s, ventral blood-sinus.

This organ corresponds to the hypopharyngeal groove of Amphioxus and the median part of the thyroid gland of Vertebrata. It is interesting to notice that the (at least) four longitudinal tracts of gland-cells are of remarkable constancy, being found not only in all groups of Tunicata, including even the pelagic, tailed Appendicularians, but also in Amphioxus and in the young thyroid gland of the Ammocoete. When, in Ascidians, a third marginal glandular tract is added it has a different appearance from the two characteristic tracts. The mucus is carried forward by the action of the large floor-cilia of the endostyle (Fig. 20) to the groove between the peripharyngeal bands, and after encircling the anterior end of the branchial sac and collecting the food particles, it passes backwards along the dorsal edge of the branchial sac to the oesophagus, guided by a membranous fold, the dorsal lamina (Fig. 21, d.l), which is more or less ridged or corrugated, and may be armed with marginal tags or even replaced by larger processes (the "languets") in some species of Ascidians. In the living animal the lamina has its free edge curved to the right hand side in such a manner as to constitute a fairly perfect tube along which the train of food passes.

Fig. 21.—Antero-dorsal part of pharynx in Ascidia mentula, × 15. br.s, Part of branchial sac; d.l, dorsal lamina; d.t, dorsal tubercle; p.br.z, prebranchial zone; p.p, peripharyngeal bands; sph, sphincter of branchial aperture; tn, tentacle.

Branchial Sac.—Thus we have the dorsal lamina (or the languets) along the dorsal edge, the endostyle along the ventral edge, and the peripharyngeal bands around the anterior end. The wall of the branchial sac itself is penetrated by a large number of channels through which blood flows. Some of these run in one direction and some in another, so as to form complicated networks, which differ greatly in their arrangement in different Ascidians. Between these blood-channels there are clefts ("stigmata"), the secondary or subdivided gill-slits, by means of which the current of water passes from the branchial sac to the large external peribranchial or atrial cavity. All the stigmata (of which there may be several hundred thousand) in the wall of the branchial sac are bounded by cubical or columnar epithelial cells, which are ciliated. These cilia, so long as the animal is alive, are in constant motion, so as to drive the water onwards, and it is this constant ciliary action in the walls of the branchial sac that gives rise to the all-important current of water streaming through the body. In addition to the stigmata there are generally one or two much larger elongated slits (Garstang's pharyngo-cloacal slits) placed close to the dorsal lamina and leading direct to the cloaca.

Fig. 22.—A mesh of the branchial sac of Ascidia, seen A, from inside; B, in horizontal section. c.d, Connecting duct; h.m, horizontal membrane; i.l, internal longitudinal bars; l.v, interstigmatic vessels; p, p′, papillae; sg, stigmata; tr, transverse vessels.

Fig. 22 shows a small part of the wall of the branchial sac, in which it may be seen that the bars containing the blood-channels are arranged in three regular series:—(1) The "transverse vessels" which run horizontally round the wall and open at their dorsal and ventral ends into large median longitudinally running tubes, the dorsal blood-sinus (or "dorsal aorta") behind the dorsal lamina, and the ventral blood-sinus (or "branchial aorta") beneath the endostyle; (2) the fine longitudinal or "interstigmatic vessels" which run vertically between adjacent transverse vessels and open into them, and which therefore bound the stigmata; and (3) the "internal longitudinal bars" which run vertically, in a plane internal to that of the transverse and fine longitudinal vessels. These bars (Fig. 22, i.l) communicate with the transverse vessels by short side branches where they cross, and at these points are prolonged into the cavity of the sac in the form of hollow papillae. In some Ascidians (e.g. Corella and most of the Molgulidae) the interstigmatic vessels are curved so that the stigmata form more or less complete spirals (see Figs. 35 and 41). In some species of Ascidia, and other Ascidians, the interstigmatic vessels are inserted into the transverse vessel in an undulating course in place of the straight line seen in Fig. 22, B, l.v, the result being that the stigmatic part of the wall of the branchial sac seems to be folded or thrown into microscopic crests and troughs. This is known as "minute plication." In some cases, again (Cynthiidae), the whole wall of the sac is pushed inwards at intervals to form large folds visible to the eye (see Fig. 36, A and B). The intersections of the internal longitudinal bars with the transverse vessels divide up the inner surface of the branchial sac wall into rectangular areas called "meshes." One such mesh, containing eight stigmata in a row, is seen in Fig. 22, A. The internal longitudinal bars bear papillae at the angles of the meshes, and occasionally in intermediate positions. There are frequently horizontal membranes (Fig. 22, B, h.m) attached to the transverse vessels between the papillae. There are many "connectives" running from the outer wall of the branchial sac to the mantle outside, and allowing the blood in the transverse vessels to communicate with that in the sinuses of the mantle (see Fig. 19, con).

Heart and Circulation.—It is one of the notable features of the Tunicata that the circulation is not constant in direction, but is periodically reversed.

The blood of Ascidians is in the main transparent, but usually contains certain pigmented corpuscles in addition to many ordinary leucocytes or colourless amoeboid cells. The pigment in the coloured cells may be red, yellow, brown, or in some cases blue or opaque white. The blood may reach the branchial sac either from the dorsal or from the ventral median sinus according to the direction in which the heart is beating at the moment (see below); and it is a most interesting and beautiful sight to see the circulation of the variously coloured corpuscles through the transparent vessels, and the lashing of the cilia along the edges of the neighbouring stigmata in a small Ascidian under the microscope.

In Ascidia (Fig. 23) the heart is an elongated fusiform tube placed on the ventral and posterior edge of the stomach, projecting into a space (the pericardium) which is a part of the original coelom, the remainder of which is represented in the adult by the reproductive and renal cavities. The wall of the heart is continuous along one edge with that of the pericardium, and the heart is to be regarded as a tubular invagination of the pericardial wall, shutting in a portion of the surrounding space (the blastocoel of the embryo), and having open ends which communicate with the large blood sinuses leading to the branchial sac, to the viscera, and to the body-wall and test. The cavity of the heart is not divided and there are no valves. Its wall is formed of a single layer of epithelio-muscular cells, the inner, muscular, ends of which are cross-striated fibres running round the heart—the only striated muscular tissue found in the body. Waves of contraction pass along the heart from end to end, first for a certain number of beats in one direction, and then, after an interval, in the other. If a small or young Ascidia be placed alive, left side uppermost, in a watch-glass or small trough of sea-water, and examined with a low power of the microscope, the heart will be readily seen near the posterior end of the transparent body. It will be noticed that the "beating" looks like successive waves of blood pressed through the tubular heart from one end to the other by its contractions. After watching the waves passing, let us say, from the right hand end of the heart to the left for about a minute and a half (perhaps 60 or 80 to 100 beats), it will be seen that they gradually become slower and then stop altogether. But after seven or eight seconds a faint wave of contraction will start from the left end of the heart and pass over it to the right; and this will be followed by larger ones for a minute and a half, and then again a pause will occur and the direction change. It has been suggested that the cause of this remarkable reversal may possibly be that the heart being on the ventral vessel, which is wider than the corresponding dorsal trunk, pumps the blood into either the lacunae of the branchial sac or those of the viscera in greater volume than can possibly get out through the smaller branchio-visceral vessel in the same time, the result being that the lacunae in question soon become engorged, and by back pressure cause the stoppage, and then reversal of the beat. The absence of any valves in the heart to regulate the direction of flow obviously facilitates this alternation of the current.

The larger channels through which the blood flows may be lined with a delicate endothelium, but the smaller passages are merely spaces in the connective tissue. The heart, although anatomically a "ventral vessel," runs in the main dorso-ventrally. The blood-channel leaving the ventral end of the heart is the "branchio-cardiac vessel" (Fig. 23, b.c). This gives off a branch which, along with a corresponding branch from the "cardio-visceral" vessel (c.v) at the other end of the heart, goes to the test, and then runs along the ventral edge of the branchial sac as the branchial aorta (b.a), external to the endostyle, communicating laterally with the ventral ends of all the transverse vessels of the branchial sac. The cardio-visceral vessel (Fig. 23, c.v) after giving off its branch to the test breaks up into a number of sinuses which ramify over the alimentary canal and the other viscera. These visceral lacunae finally communicate with a third great sinus, the "branchio-visceral" vessel (b.v) which runs forward along the dorsal edge of the branchial sac as the dorsal aorta (d.a), externally to the dorsal lamina, and joins the dorsal ends of all the transverse vessels of the branchial sac. Besides these three chief systems—the branchio-cardiac, the cardio-visceral, and the branchio-visceral—(see Fig. 23), there are numerous lacunae in all parts of the body by means of which anastomoses are established between the different currents of blood.

Fig. 23.—Diagrammatic dissection of Ascidia, from left side, to show course of circulation. Front part of branchial sac opened, back part covered by viscera. b.a, Branchial (ventral) aorta; b.c, branchio-cardiac vessel; b.v, branchio-visceral vessel; c.v, cardio-visceral vessel; d.a, dorsal aorta; ht, heart. A, anterior; P, posterior; D, dorsal; V, ventral.

When the heart contracts ventro-dorsally the course of the circulation is as follows:—the blood which is flowing through the vessels of the branchial sac is collected in an oxygenated condition in the branchio-cardiac vessel, and after receiving a stream of blood from the test enters the ventral end of the heart. It is then propelled from the dorsal end into the cardio-visceral vessels, and so reaches the test and the digestive and other viscera; then, after circulating in the visceral lacunae it passes into the branchio-visceral vessel in an impure condition, and is distributed to the branchial vessels to be purified again. When the heart, on the other hand, contracts dorso-ventrally, this course of the circulation is reversed, the "veins" and "arteries" exchange functions, and what a minute before was a "systemic," is now a "respiratory" heart. This is a phenomenon without parallel in the animal kingdom.

All the blood-spaces and lacunae are probably derived, like the cavity of the heart, from the blastocoel of the embryo, and are not, like the cavity of the pericardium, a part of the coelom (of endodermal origin).

Neural Gland and Dorsal Tubercle.—In the dorsal median line near the anterior end of the body, and imbedded in the mantle on the ventral[^[95]] surface of the nerve-ganglion, there lies a small glandular mass—the neural gland—which, as Julin first showed, there is some reason to regard as the homologue of the hypophysis cerebri of the Vertebrate brain. Metcalf has recently shown that the neural gland may be a double structure—partly cerebral and partly stomodaeal—as in Vertebrates.

Fig. 24.—Antero-dorsal part of Ascidia showing the relations of the layers of the body, and of the nervous system. A, in sagittal section; B, in transverse section. d.bl.s, Dorsal blood-sinus; d.l, dorsal lamina; d.n, dorsal nerve; d.t, dorsal tubercle; ect, ectoderm; en, endoderm; e.p.br, epithelium of peribranchial cavity; gl.d, duct of subneural gland; l.v points to the ciliated epithelium covering a longitudinal vessel of branchial sac; m, mantle; n, nerve; n.g, ganglion; n.gl, neural gland; p.br, peribranchial cavity; pp.b, peripharyngeal bands; sph, branchial sphincter; t, t′, test; tn, tentacle.

The function of this gland is still somewhat mysterious. It may merely form the viscid secretion which is carried along the peripharyngeal bands and down the dorsal lamina. On the other hand, it has been suggested that the function of the organ may possibly be renal, for the removal of nitrogenous waste matters in the neighbourhood of the nervous system. Finally, it may be a lymph gland.

The neural gland, which was first noticed by Hancock, may be continued backwards along with the dorsal nerve, and it communicates anteriorly by means of a narrow duct with the front of the branchial sac (pharynx). The opening of the duct is enlarged to form a funnel-shaped cavity (Fig. 24, A), which may be folded upon itself, convoluted, or even broken up into a number of smaller openings (see Fig. 43, p. [79]), so as to form a complicated projection called the dorsal tubercle, situated in the dorsal part of the prebranchial zone. The dorsal tubercle in Ascidia mentula is somewhat horse-shoe shaped (Fig. 21, d.t); it varies in most Ascidians (see Fig. 43) according to the genus and species, and in some cases in the individual also. Sensory cells are found in the epithelium, and so it is highly probable that besides being the opening of the duct from the neural gland, this convoluted ciliated ridge may be a sense-organ for testing the quality of the water entering the branchial sac.

Nervous System and Sense-Organs.—The single elongated ganglion (Fig. 24, n.g), in the median dorsal line of the mantle, between the branchial and atrial siphons, is the only nerve-centre in Ascidia and most other Tunicata. It is the degenerate remains of the dorsal wall of the tubular cerebro-spinal nervous system of the trunk-region of the tailed larval Ascidian—the ventral wall opposite having given rise to the subneural gland. The more posterior or spinal part of the larva has almost entirely disappeared in most adult Tunicata. It persists, however, in the Appendiculariidae, and traces of it have been found in the dorsal nerve running backwards towards the oesophagus in some Ascidians (e.g. Clavelina). It may be ganglionated in Molgulidae.

The ganglion has small rounded nerve-cells on its surface, and interlacing nerve-fibres inside. It gives off distributory nerves at both ends (Fig. 24, A), which run through the mantle to the neighbourhood of the apertures, where they divide up to supply the lobes and the sphincter muscles. The only sense-organs are the pigment spots ("ocelli," formed of modified ectoderm cells imbedded in red and yellow pigment), between the branchial and atrial lobes, the tentacles at the base of the branchial siphon, and probably the dorsal tubercle and the languets or dorsal lamina, in all of which, as well as in the endostyle and peripharyngeal bands and in papillae on the ectoderm and in the branchial sac, sensory cells have been found. These, considered as sense-organs, are all in a lowly-developed condition. The larval Ascidians, on the other hand, have well-developed intra-cerebral optic and otic sense-organs (see Fig. 26, p. [60]), and in some of the pelagic Tunicata, otocysts and pigment-spots are found in connexion with the ganglion.

Alimentary Canal.—The mouth and pharynx (branchial sac) have already been described. The remainder of the alimentary canal is a bent tube, which in A. mentula and most other Ascidians lies imbedded in the mantle on the left side of the body, and projects into the peribranchial cavity (see Figs. 18 and 19). The oesophagus leaves the branchial sac in the dorsal middle line, near the posterior end of the dorsal lamina. It is a short curved tube which leads ventrally to the large fusiform thick-walled stomach, ridged internally. The intestine emerges from the ventral end of the stomach and soon turns anteriorly, then dorsally, and then posteriorly, so as to form a curve, the intestinal loop, in which the ovary lies, open posteriorly. The intestine now curves anteriorly again, and from this point runs nearly straight forward as the rectum, thus completing a second curve, the rectal loop, in which the renal vesicles lie, open anteriorly. The wall of the intestine is thickened internally to form the typhlosole (Fig. 18, ty), a pad which runs along its entire length, so as to reduce the lumen of the tube to a crescentic slit. The anus opens into the dorsal or cloacal part of the peribranchial cavity near the atrial aperture. The walls of the stomach are glandular, and most of the endoderm cells lining the tube are ciliated. A system of delicate, microscopic, branched tubules with dilated ends (the "refringent organ"), which ramifies over the outer wall of the intestine, and communicates with the cavity of the stomach at the pyloric end by means of a duct is probably a digestive gland. There is in Ascidia no separate large gland to which the name "liver" can be applied, as in some other Tunicata.

Renal Organ.—A mass of large clear-walled vesicles which occupies the rectal loop (Figs. 18 and 19, ren), and may extend over the adjacent walls of the intestine, is a renal organ without a duct. Each vesicle is the modified remains of a part of the primitive coelom or body-cavity, and is formed of cells which eliminate nitrogenous waste matters from the blood circulating in the neighbouring blood-lacunae, and deposit them in the cavity of the vesicle, where they form one or more concentrically laminated concretions of a yellowish or brownish colour, sometimes coated with a chalky deposit. These concretions contain uric acid, and in a large Ascidian are very numerous. The nitrogenous waste products are thus deposited and stored up in the renal vesicles in place of being excreted from the body. In other Ascidians the renal organs may differ from the above in position and structure; but in no case have they any excretory duct, unless the neural gland is to be regarded as one of the renal organs—which has not yet been proved.

Reproductive Organs.—Ascidia mentula is hermaphrodite, and the reproductive organs lie with the alimentary canal, on the left side of the body (Fig. 19, ov). The ovary is a ramified gland which occupies the greater part of the intestinal loop. It contains a cavity which, along with the cavities of the testis, is derived from an embryonic coelom; the ova are formed from its walls, and fall when mature into the cavity. The oviduct is continuous with the cavity of the ovary, and leads forward alongside the rectum, finally opening near the anus into the peribranchial cavity (Fig. 18, g.d). The testis is composed of a great number of delicate, branched tubules, which ramify over the ovary and the adjacent parts of the intestinal wall. These tubules terminate in ovate swellings. Near the commencement of the rectum the larger tubules unite to form the vas deferens, a tube of considerable size, which runs forward alongside the rectum, and, like the oviduct, terminates by opening into the peribranchial cavity close to the anus. The lumen of the tubules of the testis, like the cavity of the ovary, is a part of the embryonic mesoblastic space, and the spermatozoa are formed from the cells lining the wall. In some Ascidians (certain Molgulidae and Cynthiidae), reproductive organs are present on both sides of the body, and in others, as in Polycarpa, there are many complete sets of both male and female systems attached to the inner surface of the mantle on both sides of the body and projecting into the peribranchial cavity.

Embryology and Life-History of a Typical Ascidian.

The eggs of Tunicata are for the most part of small size, nearly colourless and transparent, and with little or no food-yolk. In some, however (such as some of the Cynthiidae, and some Compound Ascidians), the eggs are larger, more opaque, and have a fair amount of food-yolk. Ova of this type are not expelled from the body of the parent as ova, but are fertilised, and remain in the atrial cavity or in a special diverticulum thereof—the incubatory pouch—until they are far advanced in development; and usually leave the body as tailed larvae. In many species, the ova and spermatozoa mature at different times in the life-history, and so self-fertilisation is prevented. Some species (such as many Botryllidae and Distomatidae) are protogynous, the ova being produced and shed before the testes have matured, while other species (Coelocormus huxleyi) are protandrous, being male while young and female later. But there is no doubt that in other cases (e.g. Ascidia mentula) self-fertilisation is not only possible, but does take place. After maturation certain of the follicle-cells which invest the ovum in the ovary migrate into the egg and proliferate so as to form a layer in the superficial part of the egg, where they appear as the so-called "testa-cells" or "kalymmocytes" (Fig. 25, A, t.c). The remaining follicle-cells may form two or more layers, usually one of large cubical cells, which may become greatly vacuolated, next to the ovum, and an external flattened layer which is cast off when the egg escapes from the ovary.

Segmentation is complete and results in the formation of a spherical blastula with a small segmentation-cavity (Fig. 25, C). The blastula grows larger and begins to differentiate.[^[96]] There are slightly smaller cells which divide more rapidly at one end of this embryo, the future ectoderm, and slightly larger and more granular cells at the other, which become chiefly endoderm (hypoblast). Invagination of the larger cells then takes place (Fig. 25, D), resulting in the formation of a gastrula with an archenteron. The hypoblast cells lining the archenteron become columnar (hy). The curving and more rapid growth at the anterior end of the embryo narrow the primitively wide open blastopore, and carry it to the posterior end of the future dorsal surface (Fig. 25, E). The orientation of the body is now clear.

Fig. 25.—Embryology of Ascidian. A, mature ovum: foll, follicle-cell; m, membrane; n, nucleus; p, protoplasm; t.c, test-cell; B, mature spermatozoon; C, segmentation-stage in section to show blastocoel; D, early gastrula-stage; E, later gastrula-stage; F, later embryo showing rudiments of notochord and neural tube; G, transverse section of body of embryo showing mesoblast and formation of neural canal; H, late embryo showing body and tail, notochord, neural canal, and mesenteron; I, young larva ready to be hatched; K, transverse section of tail of larva. ar, Archenteron; at, atrial invagination; au, otocyst; b.c, blastocoel; b.p, blastopore; ch, notochord; ep, epiblast; f, tail-fin; hy, hypoblast; m.b, mesoblast; mes, mesenteron; musc, muscle-cell; n.c, neural canal; ne.c, neurenteric canal; n.v, neural vesicle; oc, ocellus. (Modified from Kowalevsky and others.)

The embryo is elongated antero-posteriorly, the dorsal surface is flattened, and the blastopore indicates its posterior end. Around the blastopore the large ectoderm cells form a medullary plate, along which a groove (the medullary groove), runs forwards, bounded at the sides by medullary folds which meet behind the blastopore. Underneath the posterior part of the medullary groove certain of the hypoblast cells from the dorsal wall of the archenteron, in the median line, form a band extending forwards (Fig. 25, E, ch). This band separates off from the hypoblast, which closes in beneath it, and thus gives rise to the notochord (Fig. 25, F). The more lateral and posterior cells become mesoblast, and separate off as lateral plates, which show no trace of metameric segmentation (Fig. 25, G). The remainder of the archenteron becomes the branchial sac, and by further growth buds off the rest of the alimentary canal.

The medullary groove now becomes converted into the closed neural canal by the growing up and arching inwards (Fig. 25, G, n.c) of the medullary folds, which unite with one another from behind forwards in such a way that the blastopore now opens from the enteron into the floor of the neural canal, forming the neurenteric passage (Fig. 25, F, n.e.c). For a time the anterior end of the neural canal remains open as a neuropore. By this time the posterior end is elongating to form a tail, and the embryo is acquiring the tadpole-shape (Fig. 25, H) characteristic of the free larva. The tail grows rapidly, curves round the body, and also undergoes torsion, so that its dorsal surface comes to lie on the left side. It contains ectoderm cells on its surface, notochordal cells (in single file) up the centre (see Fig. 25, H, ch), a neural canal dorsally, and a row of endoderm cells representing the enteron ventrally to the notochord. Later on the mesoblast also is prolonged into the tail, where it forms a band of striated muscle-cells at each side of the notochord. When the ectoderm cells begin to secrete the cuticular test this forms two delicate transparent longitudinal (dorsal and ventral) fins in the tail (Fig. 25, K, f), and especially at its extremity where radial thickenings form striae resembling fin-rays. The ectoderm on the anterior end of the body grows out into three adhering papillae (Fig. 26, A).

The neural canal now differentiates into a tubular dorsal nervous system. The anterior end dilates to form the thin-walled cerebral vesicle (see Figs. 25, I, and 26, A), containing later the intra-cerebral, dorsal, pigmented eye (oc), and the ventral otolith (au) of the larva. The next part of the canal thickens to form the trunk-ganglion, and behind that is the more slender "spinal cord," which runs to the extremity of the tail. A ciliated diverticulum of the anterior end of the enteric cavity (future pharynx) which enters into close relations with the front of the cerebral vesicle,[^[97]] and later opens into the ectodermic invagination which forms the mouth at that spot, is evidently the rudiment of the neural duct or hypophysial canal. The future branchial sac (pharynx), with a ventral median thickening which will be the endostyle, is by this time clearly distinguishable by its large size from the much narrower posterior part of the enteron, which grows out to become the oesophagus, stomach, and intestine. The notochord does not extend forward into the pharyngeal region, but is confined to the posterior or caudal part of the embryo. It now shows lenticular pieces of a gelatinous intercellular substance secreted by the cells and lying between them (Fig. 25, I). The mouth forms as a stomodaeum, or ectodermal invagination, antero-dorsally in the region where the neuropore has closed, and about the same time two lateral ectodermal involutions form (Fig. 26, A, at), which become the atrial or peribranchial pouches, at first distinct, afterwards united in the mid-dorsal line to form the adult cloaca and atrial aperture. Ingrowths from the atrial pouches and outgrowths from the wall of the pharynx coalesce to form the proto-stigmata (primary gill-slits) by which the cavity of the branchial sac is first placed in communication with the exterior through the atrial apertures. Opinions differ as to whether only one or a few pairs of true gill-clefts are represented in the young Ascidian; and the actual details of their formation and subdivision, to form the stigmata of the adult, differ considerably in different forms. In Clavelina the stigmata are formed as independent perforations of the pharyngeal wall; in Ascidia two pairs of protostigmata increase to six pairs, which are subdivided into stigmata; Botryllus and other forms are intermediate in some respects. No doubt the subdivision of proto-stigmata is primitive, but has been lost from the ontogeny in some cases. To what precise extent the walls of the atrial or peribranchial cavities are formed of ectoderm, or of endoderm, is still doubtful.

The embryo is hatched about two or three days after fertilisation, as a larva or Ascidian tadpole (Fig. 26, A) which leads a free-swimming existence for a short time, during which it develops its nervous system and cerebral sense-organs, and the powerful mesoblastic muscle-bands lying at the sides of the notochord (now a cylindrical rod of gelatinous nature surrounded by the remains of the original cells) in the tail which form the locomotory apparatus. Fig. 26, A, shows this stage, the highest in its chordate organisation, when the larva swims actively through the sea by vibrating its long tail with the dorsal and ventral fins.

In addition to the structures already mentioned, the mesoderm has formed the beginning of the muscular body-wall, the connective tissue around the organs, and the blood; the endostyle has developed as a thick-walled groove along the ventral edge of the pharynx, which has become the branchial sac; and the pericardial sac and its invagination the heart have formed in the mesoblast between the endostyle and stomach. The "epicardiac tubes" grow out from the posterior end of the endostyle to join the pericardium. They play an important part in the formation of buds in the colonial Tunicata. The heart acquires a connexion with blastocoelic blood-spaces at its two ends. The heart and pericardium show the same relations in Tunicata as in Enteropneusta, but it is very doubtful whether these organs are genetically related to the Vertebrate heart.

Fig. 26.—Metamorphosis of an Ascidian. A, free-swimming tailed larva; B, the metamorphosis—larva attached; C, tail and nervous system of larva degenerating; D, further degeneration and metamorphosis of larva into E, the young fixed Ascidian. at, Atrial invagination; ch, notochord; hy, hypoblast cells; i, intestine; m, mouth; mes, mesenteron; n.c, neural canal; n.v, neural vesicle with sense-organs. (Modified from Kowalevsky and others.)

The unpaired optic organ in the cerebral vesicle when fully formed has a retina, pigment layer, lens and cornea; while the ventral median organ is a large, spherical, partially-pigmented otolith attached by delicate hair-like processes to the summit of a hollow "crista acustica" (Fig. 26, A). After a few hours, or at most a day or so, the larva attaches itself by one or more of the three anterior ectodermal glandular papillae (one dorsal and two lateral) to some foreign body, and commences the retrogressive metamorphosis which leads to the adult state. The adhering papillae, having performed their function, begin to atrophy, and their place is taken by the rapidly increasing test. The tail which at first vibrates rapidly is partly withdrawn from the test and absorbed, and partly cast off in shreds (Fig. 26, B, C, D). The notochord, nerve-tube, muscles, etc., are withdrawn into the body, where they break down and are absorbed by phagocytes. The posterior part of the nerve cord and its anterior end with the large sense-organs disappear, and the middle part or trunk-ganglion is reduced to form the relatively small ganglion of the adult, underneath which the hypophysial tube gives rise to the neural gland. While the locomotory, nervous and sensory organs are thus disappearing, or being reduced, the alimentary canal and reproductive viscera are growing largely. The branchial sac enlarges, its walls become penetrated by blood-channels, and grow out to form bars and papillae, and the number of openings greatly increases by the primary gill-slits being broken up into the transverse rows of stigmata. The stomach and intestine, which developed as an outgrowth from the back of the branchial sac at the right side, become longer and curve, so that the end of the intestine acquires an opening into at first the left hand side, and eventually the cloacal or median part of the atrial cavity. The adhering papillae have now disappeared, and are replaced functionally by a growth of the test over neighbouring objects; and at the same time the region of the body between the point of fixation and the mouth (branchial aperture) increases rapidly in extent, so as to cause the body of the Ascidian to rotate through about 180°, and thus the branchial siphon is carried to the opposite end from the area of attachment (see Fig. 26, B, C, D, E). Finally the gonads and their ducts form in the mesoderm between the stomach and intestine. We thus reach the sedentary degenerate fixed adult Ascidian with little or no trace of the Chordate characteristics so marked in the earlier larval stage (see E and A, Fig. 26). The free-swimming tailed larva shows the Ascidian at the highest level of its organisation, and is the stage that indicates the genetic relationship of the Tunicata with the Vertebrata.

In some Ascidians with more food-yolk in the egg, or in which the development takes place within the body of the parent, the life-history as given above is more or less modified and abbreviated, and in some few forms the tailed larval stage is missing. Some exceptional cases of development will be noted below under the groups to which they belong.

The remarkable life-history of the typical Ascidian, of which the outlines are given above, is of importance from two points of view:—

1. It is an excellent example of degeneration. The free-swimming larva is a more highly developed animal than the adult Ascidian. The larva is, as we have seen, comparable with a larval fish or a young tadpole, and is thus a Chordate animal showing evident relationship to the Vertebrata; while the adult is in its structure non-Chordate, and is on a level with some of the worms, or with the lower Mollusca, in its organisation, although of an entirely different type.

2. It shows us the true position of the Ascidians (Tunicata) in the animal series. If we knew only the adult forms we might regard them as being an aberrant group of Worms, or possibly as occupying a position between worms and the lower Mollusca, or we might place them as an independent group; but we should certainly have to class them as Invertebrate animals. But when we know the whole life-history, and consider it in the light of "recapitulation" and "evolutionary" views we recognise that the Ascidians are evidently related to the Vertebrata, and were at one time free-swimming Chordata occupying a position somewhere below the lowest Fishes.

CHAPTER III

TUNICATA (CONTINUED)

CLASSIFICATION: LARVACEA—APPENDICULARIANS—STRUCTURE, ETC.—ASCIDIACEA—SIMPLE ASCIDIANS—SPECIFIC CHARACTERS—COMPOUND ASCIDIANS—GEMMATION—MEROSOMATA—HOLOSOMATA—PYROSOMATIDAE—THALIACEA—DOLIOLIDAE—SALPIDAE—GENERAL CONCLUSIONS—PHYLOGENY.

We now turn to the systematic classification of the group; and further details of structure or function, points of interest in the life-history such as budding and the formation of colonies, the habits and occurrence, and other peculiarities such as phosphorescence, will all be noted under the orders, sub-orders, families and genera in which they occur.

CLASS TUNICATA.

The Tunicata or Urochordata are hermaphrodite marine Chordate animals, which show in their development the essential Vertebrate characters, but in which the notochord is restricted to the posterior part of the body, and is in most cases present only during the free-swimming larval stages. The adult animals are usually sessile and degenerate, and may be either solitary or colonial, fixed or free. The nervous system is, in the larva, of the elongated, tubular, dorsal, Vertebrate type, but in most cases it degenerates in the adult to form a small ganglion placed above the pharynx. The body is completely covered with a thick cuticular test ("tunic") which contains a substance similar to cellulose. The alimentary canal has a greatly enlarged respiratory pharynx or branchial sac, which is perforated by two or many more or less modified gill-slits opening into a peribranchial or atrial cavity, which communicates with the exterior by a single dorsal exhalent aperture (rarely two ventral apertures). The ventral heart is simple and tubular, and periodically reverses the direction of the blood-current.

Fig. 27.—Sketch of the chief kinds of Tunicata found in the sea.

This Class is divided into three Orders:—The Appendicularians, the Ascidians, and the Salpians (see Fig. 27).

Order I. Larvacea (Appendicularians).

Free-swimming pelagic forms, in which the posterior part of the body takes the form of a large locomotory appendage, the "tail," in which there is a skeletal axis, the urochord. A relatively large cuticular test, the "house," may be formed with great rapidity (in an hour or so) as a secretion from a part of the ectoderm; it is, however, merely a temporary structure which is soon cast off and replaced by another. The branchial sac is simply an enlarged pharynx with two ventral ciliated openings (stigmata) leading to the exterior. These may be regarded as the representatives of the primary gill-slits (undivided) of the Ascidian. There are thus a single pair. There is no separate peribranchial, atrial, or cloacal cavity. The nervous system consists of a large dorsally placed ganglion and a long nerve-cord, which stretches backwards over the alimentary canal to reach the tail, along which it runs on the left side (morphological dorsal edge) of the urochord. The anus opens ventrally on the surface of the body, usually in front of the stigmata. No reproduction by gemmation or metamorphosis is known in the life-history.

Structure and Mode of Life.—This is one of the most interesting groups of the Tunicata, as it shows more completely than any of the rest the probable characters of the ancestral forms. It has undergone little or no degeneration, and consequently corresponds more nearly to the tailed, larval condition than to the adult forms of the other groups. It retains, in fact, the originally posterior, chordate, part of the body which is lost in the metamorphosis of all the other Tunicata. Hence the Appendicularians have been described as permanent, or sexually mature, larval forms, and hence also the adult Ascidia may be said to correspond to the trunk alone of the Appendicularian. The Order includes a single group, the Appendiculariida, all the members of which are minute (usually about 5 mm. in total length) and free-swimming (Fig. 28). They occur near the surface of the sea (and exceptionally in deeper water) in most parts of the world, moving in a characteristic vibratory manner by the contractions of the powerful tail (see Fig. 27). They possess the power of forming with great rapidity, from tracts of specially large glandular ectoderm cells, the "oikoplasts," an enormously large (many times the size of the body) investing gelatinous layer, which probably corresponds to the test of other groups, although it is doubtful whether it contains cellulose, and it differs also in having no immigrated cells and in its temporary nature. This structure (Fig. 28) was first described by Von Mertens, and by him named "Haus"; it has recently been more minutely investigated by Lohmann. It is only loosely attached to the body, and is frequently thrown off soon after its formation. Its function is probably protective, and possibly to some extent hydrostatic, and it may also be of use in straining the nutritive particles from the large volumes of water which filter through its complicated passages and perforated folds.[^[98]] The long, laterally compressed "tail" in the Appendiculariida is attached to the ventral surface of the body (Fig. 30), and is bent downwards and forwards, so that it usually points more or less anteriorly; and is twisted through an angle of 90°, so that the dorsal edge lies to the left. It shows what have been interpreted as traces of metameric segmentation, having its lateral muscle-bands broken up into successive pieces (supposed myotomes, probably only cells), while the nerve-cord presents a series of enlargements formed of groups of nerve-cells from which distributory nerves are given off. In Oikopleura the muscle-band in the tail is formed of ten cells fused on each side. Near the base of the tail there is a distinctly larger elongated ganglion. The urochord in the tail consists of a homogeneous rod surrounded by a sheath containing nuclei.

Fig. 28.—Appendiculariida. A, Appendicularia sicula, Fol, with house; B, Megalocercus abyssorum, Chun, nat. size; C, Oikopleura cophocerca, Gegenb., with house; D, Fritillaria megachile, Fol, with vesicle; E, Appendicularian in its house; F and G, two stages in the formation of the house. (A to D from Seeliger; E to G from Lohmann.)

The anterior (cerebral) ganglion has connected with it an otocyst (Fig. 29), a pigment spot, and a tubular richly ciliated process opening into the branchial sac, and representing the dorsal tubercle and associated parts of an ordinary Ascidian. The tube ends in a plain or coiled cellular mass lying to the right of the ganglion. No neural gland is found.

Fig. 29.—Transverse section through anterior part of Oikopleura to show ganglion, sense-organs, endostyle, etc. × 300. br.s, Branchial sac; c.f, ciliated funnel; ec, dorsal ectoderm; end, closed anterior end of endostyle; hy, hypobranchial groove in floor of branchial sac; n.g, nerve-ganglion; or.gl, oral gland; ot, otocyst; x, opening of ciliated funnel into pharynx.

The branchial aperture or mouth leads into the simple branchial sac or pharynx (Fig. 30, br.s). There are no tentacles. The endostyle is short, is a closed tube both anteriorly and posteriorly (Fig. 29), and has about four longitudinal rows of gland-cells. There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly to unite close in front of the oesophageal opening. The wall of the branchial sac does not show the complex structure usual in Tunicata, and has only two ciliated apertures (Figs. 30, 31, 32, sg). These are homologous with the primary stigmata of the typical Ascidians, and with a pair of the gill-clefts of Vertebrates. They are placed far back on the ventral surface, one on each side of the middle line, and lead into short funnel-shaped tubes which open on the surface of the body behind the anus (Fig. 30, at). These tubes correspond to the right and left atrial involutions, which in an ordinary Ascidian fuse to form the peribranchial cavity. The remainder of the alimentary canal consists of oesophagus, stomach (which may have a glandular diverticulum), intestine and rectum (Fig. 30). The heart, surrounded ventrally by a delicate pericardial membrane, lies below and in front of the stomach, and is formed by the differentiation of the outer ends of epithelial cells into muscular fibrillae. Two specially large glandular cells are placed at the opposite ends of the heart. There are no blood-vessels except the remains of the primary body-cavity (blastocoel). No heart can be seen in some of the smaller species of Oikopleura. Nearly all the species are hermaphrodite, and the large ovary and testis are placed at the posterior end of the body. There is no proper oviduct, the genital products merely breaking through to the exterior at the point marked g.d in Fig. 30. The spermatozoa are generally matured and shed before the ova, and thus self-fertilisation is prevented. The ova are very small, and little is known of the development.

Fig. 30.—Longitudinal optical section of Oikopleura. Part of the tail is cut off. a, Anus; at, atrial opening; br.s, branchial sac; c.f, ciliated funnel; ec, ectoderm; end, endostyle; ep.p, epipharyngeal ridge; g.d, opening of gonads to exterior; ht, heart; hy.p, hypopharyngeal ridge; i, intestine; m, mouth; mus, muscle-bands in tail; n, nerve-cord; n′, nerve in tail; n.ch, urochord; n.g, nerve-ganglion; n.g′, ganglion in tail; oes, oesophagus; or.gl, oral gland; ot, otocyst; ov, ovary; sg, stigmata; so, sense-organ; sp, testis; st, stomach; t, test. (After Herdman.)

Classification.—There are two Families of Larvacea: First, the Kowalevskiidae, including only the remarkable genus Kowalevskia, Fol, in which the heart and endostyle are absent, and the branchial sac is provided with four rows of ciliated tooth-like processes. The two known species have been found in the Mediterranean and in the Atlantic.

The second family Appendiculariidae comprises about eight genera, amongst which may be mentioned:—(1) Oikopleura, Mertens, and (2) Appendicularia, Fol, in both of which the body is short (1 or 2 mm. in length) and compact (Fig. 30), and the tail relatively long, while the endostyle is straight. (3) Megalocercus, Chun, from deep water in the Mediterranean; M. abyssorum is the largest Appendicularian known, having a total length of 3 cm.—it is of a bright red colour. (4) Fritillaria, Q. and G., in which the body is elongated (Fig. 32) and composed of anterior and posterior regions, the tail relatively short, the endostyle recurved, the stigmata opening far in front of the anus, and an ectodermal hood is formed over the front of the body.

In all nearly forty species of Larvacea are known.

Fig. 31.—Transverse section of body and tail of Oikopleura flabellum (?) at, Atrial tube; bl.s, blood-space; br.s, cavity of pharynx or branchial sac; ec, ectoderm; en, endoderm; ep.p, epipharyngeal ciliated bands; gel, gelatinous layer between ectoderm and endoderm; hy.p, hypopharyngeal ciliated band; mus, muscular tissue on inner surface of ectoderm of tail; n, nerve-cord; n′, its continuation in the tail; n.ch, notochord in tail; r, rectum; sg, one of the stigmata or ciliated openings from the branchial sac to the atrial tube; t, test (= young "house"); x, bridge of gelatinous tissue in front of stigma closing branchial sac off from atrial tube. (After Herdman.)

Occurrence.—Although for the most part transparent, and usually almost invisible in sea-water, some Appendicularians may have certain parts of the body (alimentary canal, endostyle, gonads, etc.) brilliantly pigmented (orange, violet, etc.), and may under exceptional circumstances be present in such profusion as to colour tracts of the sea. Appendicularians are widely distributed, having been found in all seas from the Arctic to the Antarctic, both round coasts and in the open ocean. Although a few species have been found at considerable depths in the Mediterranean, still in the Atlantic they are not deep-water animals, and as a group must be regarded as surface-forms. They are fairly abundant to a depth of 100 fathoms, and some few reach 1500. Species of Oikopleura and Fritillaria are frequent round the British coasts, our commonest species being probably O. dioica, Fol, and F. furcata, Moss. Young specimens appear in the plankton about February and March, and larger forms are as a rule found later in the summer. Several instances have been recorded of swarms of especially large forms, provided with massive tests (the "house"), having appeared suddenly on our coast in such abundance as to form an important element in the surface life of the sea.

Fig. 32.—Diagram of Fritillaria seen from the right side to show the elongated body, the hood, and the relative positions of anus, atrial opening, and gonads. (Compare with Oikopleura, Fig. 30.) a, Anus; at, opening of atrial tube; br.s, branchial sac; end, endostyle; ht, heart; m, mouth; n.ch, notochord; n.g, nerve-ganglion; oes, oesophagus; ov, ovary; sg, stigma; sp, testis; st, stomach.

Order II. Ascidiacea (Ascidians).

Fixed or free-swimming Simple or Compound Ascidians, which in the adult are never provided with a locomotory appendage or tail, and have no trace of a notochord. The free-swimming forms are colonies, the Simple Ascidians being always sedentary and usually fixed. The test is permanent and well developed, and becomes organised by the immigration of cells from the body; as a rule it increases in size with the age of the individual. The branchial sac is large and well developed. Its walls are perforated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the single atrial aperture. Many of the Ascidiacea, both fixed and free, reproduce by gemmation to form colonies, and in most of them the sexually produced embryo develops into a tailed larva.

The Ascidiacea includes three groups, the Simple Ascidians, the Compound Ascidians, and the free-swimming colonial Pyrosoma, which in some respects connects this Order with the Thaliacea.

Sub-Order 1. Ascidiae Simplices.

Fixed Ascidians, which are solitary, and very rarely reproduce by gemmation; if, as in a few cases, small colonies are formed, the members are not buried in a common investing mass, but each has a distinct test of its own. No strict line of demarcation can be drawn between the Simple and Compound Ascidians; and one of the families of the former group, the Clavelinidae (the "Social" Ascidians of Milne-Edwards), forms a transition from the typical Simple forms which never reproduce by gemmation, to the Compound forms which always do. Over 500 species of Ascidiae Simplices are now known, but there are probably very many more still undescribed. The sub-order may be divided into the following families:—

Fam. 1. Clavelinidae.—Simple Ascidians which reproduce by gemmation to form small colonies (Fig. 33), in which each member, or ascidiozooid, has a distinct test, but all are connected by a common blood-system, and by a prolongation of the "epicardiac tubes" (see p. [83]) from the branchial sac. Buds are formed on the stolons (Fig. 33), which are vascular outgrowths from the posterior end of the body, containing prolongations from the ectoderm, mesoderm, and endoderm (the epicardium) of the Ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. The Clavelinidae are the simplest of the Ascidiae Simplices. They are the forms that come nearest to the Compound Ascidians, and are closely related to the Distomatidae. They are probably the nearest representatives now existing of the ancestral forms from which both Simple and Compound Ascidians are descended.

Fig. 33.—Colony of Clavelina lepadiformis (nat. size).

This family contains amongst others the following three genera:—Ecteinascidia, Herdman, with internal longitudinal bars in the branchial sac; Clavelina, Savigny, with a long body and intestine extending behind the branchial sac (Fig. 33); and Perophora, Wiegmann, with a short compact body and intestine alongside the branchial sac. Clavelina lepadiformis and Perophora listeri are common British species found at a few fathoms depth off various parts of our coast. Both occur round the south end of the Isle of Man. In autumn Clavelina accumulates reserve-material in the ectoderm cells of parts of the stolon, which remain when the rest of the colony dies away, and then form new buds in spring.

Fam. 2. Ascidiidae.—Solitary fixed Ascidians, never forming colonies; with gelatinous or cartilaginous test; branchial aperture usually eight-lobed, atrial aperture usually six-lobed; branchial sac not folded; internal longitudinal bars usually present; stigmata straight or curved; tentacles simple; gonads in or around the intestinal loop. This family is divided into three sections:—

Sub-Fam. 1. Hypobythiinae.—Branchial sac with no internal longitudinal bars, test strengthened with curious symmetrically placed nodules.

The one genus Hypobythius, Moseley, contains two stalked deep-water forms found by the "Challenger;" H. calycodes (Fig. 34, A), from the North Pacific, 2900 fathoms, and H. moseleyi from the South Atlantic, 600 fathoms.

Fig. 34.—A, Hypobythius calycodes, Moseley; B, Chelyosoma macleayanum, Brod. and Sowb.; C, Corynascidia suhmi, Herdman; D, Rhodosoma callense, Lac.-Duth.

Sub-Fam. 2. Ascidiinae.—Internal longitudinal bars present; stigmata straight. Many genera, of which the following are the more important:—Ciona, Fleming, dorsal languets present; Ascidia, Linnaeus (in part Phallusia, Savigny), dorsal lamina present (Fig. 15, p. [40]); Rhodosoma, Ehrenberg, anterior part of test modified to form operculum (Fig. 34, D); Abyssascidia, Herdman, intestine on right side of branchial sac. The type genus of this section, Ascidia, has been described in detail above (Chapter II. p. [39]), and Figs. 15 to 26 illustrate its structure and life-history. There are many species. Ciona intestinalis, Linn. (Fig. 40, B), is one of the commonest of British Ascidians, and lives readily in aquaria.

Sub-Fam. 3. Corellinae.—Stigmata curved and forming spirals (Fig. 35). Three genera:—Corella, Alder and Hancock, test gelatinous, body sessile; Corynascidia, Herdman, test gelatinous, body pedunculated (Fig. 34, C), a remarkable deep-sea form with very delicate spirally-coiled vessels in the branchial sac (Fig. 35, A), found in the Pacific (2160 faths.) and the Southern Ocean; Chelyosoma, Brod. and Sowb., upper part of test modified into horny plates (Fig. 34, B).

Fig. 35.—A, branchial sac of Corynascidia suhmi, Herdman; B, branchial sac of Corella japonica, Herdman. i.l, Internal longitudinal bars; tr, transverse vessels. (After Herdman.)

Corella contains several British species, one of which, C. parallelogramma, O. F. Müll., is one of the commonest and most handsome Ascidians in our coralline zone (about 20 faths.). Through its clear crystalline test the lemon-yellow and carmine pigmentation of the mantle, and even (with a lens) the working of the cilia along the spiral stigmata of the branchial sac (compare Fig. 35, B), can readily be seen. The beating of the heart can be seen just in front of the viscera upon the right side of the branchial sac (compare with Ascidia, Fig. 23).