Transcriber’s Note:

New original cover art included with this eBook is granted to the public domain.

THE
CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, Sc.D., F.R.S., Fellow of King’s College, Cambridge; Keeper of the Department of Zoology in the British Museum (Natural History)

AND

A. E. SHIPLEY, M.A., Fellow and Tutor of Christ’s College, Cambridge; Reader in Zoology in the University

VOLUME IV

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CRUSTACEA

By Geoffrey Smith, M.A. (Oxon.), Fellow of New College, Oxford; and the late W. F. R. Weldon, M.A. (D.Sc., Oxon.), formerly Fellow of St. John’s College, Cambridge, and Linacre Professor of Human and Comparative Anatomy, Oxford

TRILOBITES

By Henry Woods, M.A., St. John’s College, Cambridge; University Lecturer in Palaeozoology

INTRODUCTION TO ARACHNIDA, AND KING-CRABS

By A. E. Shipley, M.A., F.R.S., Fellow and Tutor of Christ’s College, Cambridge; Reader in Zoology

EURYPTERIDA

By Henry Woods, M.A., St. John’s College, Cambridge; University Lecturer in Palaeozoology

SCORPIONS, SPIDERS, MITES, TICKS, ETC.

By Cecil Warburton, M.A., Christ’s College, Cambridge; Zoologist to the Royal Agricultural Society

TARDIGRADA (WATER-BEARS)

By A. E. Shipley, M.A., F.R.S., Fellow and Tutor of Christ’s College, Cambridge; Reader in Zoology

PENTASTOMIDA

By A. E. Shipley, M.A., F.R.S., Fellow and Tutor of Christ’s College, Cambridge; Reader in Zoology

PYCNOGONIDA

By D’Arcy W. Thompson, C.B., M.A., Trinity College, Cambridge; Professor of Natural History in University College, Dundee

MACMILLAN AND CO., LIMITED

ST. MARTIN’S STREET, LONDON

1909

All the ingenious men, and all the scientific men, and all the fanciful men, in the world, with all the old German bogypainters into the bargain, could never invent ... anything so curious, and so ridiculous, as a lobster.

Charles Kingsley, The Water-Babies.

For, Spider, thou art like the poet poor,

Whom thou hast help’d in song.

Both busily, our needful food to win,

We work, as Nature taught, with ceaseless pains,

Thy bowels thou dost spin,

I spin my brains.

Southey, To a Spider.

Last o’er the field the Mite enormous swims,

Swells his red heart, and writhes his giant limbs.

Erasmus Darwin, The Temple of Nature.

PREFACE

The Editors feel that they owe an apology and some explanation to the readers of The Cambridge Natural History for the delay which has occurred in the issue of this, the fourth in proper order, but the last to appear of the ten volumes which compose the work. The delay has been due principally to the untimely death of Professor W. F. E. Weldon, who had undertaken to write the Section on the Crustacea. The Chapter on the Branchiopoda is all he actually left ready for publication, but it gives an indication of the thorough way in which he had intended to treat his subject. He had, however, superintended the preparation of a number of beautiful illustrations, which show that he had determined to use, in the main, first-hand knowledge. Many of these figures have been incorporated in the article by Mr. Geoffrey Smith, to whom the Editors wish to express their thanks for taking up, almost at a moment’s notice, the task which had dropped from his teacher’s hand.

A further apology is due to the other contributors to this volume. Their contributions have been in type for many years, and owing to the inevitable delays indicated above they have been called upon to make old articles new, ever an ungrateful labour.

The appearance of this volume completes the work the Editors embarked on some sixteen years ago. It coincides with the cessation of an almost daily intercourse since the time when they “came up” to Cambridge as freshmen in 1880.

S. F. Harmer.

A. E. Shipley.

March 1909.

CONTENTS

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS VOLUME

The names of extinct groups are printed in italics.

TRILOBITA (p. [221]).

Families

Agnostidae (p. [244]).

Shumardiidae (p. [245]).

Trinucleidae (p. [245]).

Harpedidae (p. [245]).

Paradoxidae (p. [246]).

Conocephalidae = Conocoryphidae (p. [247]).

Olenidae (p. [247]).

Calymenidae (p. [247]).

Asaphidae (p. [249]).

Bronteidae (p. [249]).

Phacopidae (p. [249]).

Cheiruridae (p. [250]).

Proëtidae (p. [251]).

Encrinuridae (p. [251]).

Acidaspidae (p. [251]).

Lichadidae (p. [252]).

Orders.

TARDIGRADA (pp. [258], [477]).

PENTASTOMIDA (pp. [258], [488]).

PYCNOGONIDA = PODOSOMATA = PANTOPODA (p. [501]).

Families.

Decolopodidae (p. [531]).

Colossendeidae = Pasithoidae (p. [532]).

Eurycididae = Ascorhynchidae (p. [533]).

Ammotheidae (p. [534]).

Rhynchothoracidae (p. [535])

Nymphonidae (p. [536]).

Pallenidae (p. [537]).

Phoxichilidiidae (p. [538]).

Phoxichilidae (p. [539]).

Pycnogonidae (p. [539]).

CRUSTACEA

CHAPTERS I and III-VII

BY

GEOFFREY SMITH, M.A. (Oxon.)

Fellow of New College, Oxford

CHAPTER II

BY

The Late W. F. R. WELDON, M.A. (D.Sc. Oxon.)

Formerly Fellow of St. John’s College, Cambridge, and Linacre Professor of Human and Comparative Anatomy, Oxford

CHAPTER I
CRUSTACEA—GENERAL ORGANISATION

The Crustacea are almost exclusively aquatic animals, and they play a part in the waters of the world closely parallel to that which insects play on land. The majority are free-living, and gain their sustenance either as vegetable-feeders or by preying upon other animals, but a great number are scavengers, picking clean the carcasses and refuse that litter the ocean, just as maggots and other insects rid the land of its dead cumber. Similar to insects also is the great abundance of individuals which represent many of the species, especially in the colder seas, and the naturalist in the Arctic or Antarctic oceans has learnt to hang the carcasses of bears and seals over the side of the boat for a few days in order to have them picked absolutely clean by shoals of small Amphipods. It is said that these creatures, when crowded sufficiently, will even attack living fishes, and by sheer press of numbers impede their escape and devour them alive. Equally surprising are the shoals of minute Copepods which may discolour the ocean for many miles, an appearance well known to fishermen, who take profitable toll of the fishes that follow in their wake. Despite this massing together we look in vain for any elaborate social economy, or for the development of complex instincts among Crustacea, such as excite our admiration in many insects, and though many a crab or lobster is sufficiently uncanny in appearance to suggest unearthly wisdom, he keeps his intelligence rigidly to himself, encased in the impenetrable reserve of his armour and vindicated by the most powerful of pincers. It is chiefly in the variety of structure and in the multifarious phases of life-history that the interest of the Crustacea lies. Before entering into an examination of these matters, it will be well to take a general survey of Crustacean organisation, to consider the plan on which these animals are built, and the probable relation of this plan to others met with in the animal kingdom.

The Crustacea, to begin with, are a Class of the enormous Phylum Arthropoda, animals with metamerically segmented bodies and usually with externally jointed limbs. Their bodies are thus composed of a series of repeated segments, which are on the whole similar to one another, though particular segments may be differentiated in various respects for the performance of different functions. This segmentation is apparent externally, the surface of a Crustacean being divided typically into a number of hard chitinous rings, some of which may be fused rigidly together, as in the carapace of the crabs, or else articulated loosely.

Each segment bears typically a pair of jointed limbs, and though they vary greatly in accordance with the special functions for which they are employed, and may even be absent from certain segments, they may yet be reduced to a common plan and were, no doubt, originally present on all the segments.

Passing from the exterior to the interior of the body we find, generally speaking, that the chief system of organs which exhibits a similar repetition, or metameric segmentation, is the nervous system. This system is composed ideally of a nervous ganglion situated in each segment and giving off peripheral nerves, the several ganglia being connected together by a longitudinal cord. This ideal arrangement, though apparent during the embryonic development, becomes obscured to some extent in the adult owing to the concentration or fusion of ganglia in various parts of the body. The other internal organs do not show any clear signs of segmentation, either in the embryo or in the adult; the alimentary canal and its various diverticula lie in an unsegmented body-cavity, and are bathed in the blood which courses through a system of narrow canals and irregular spaces which surround all the organs of the body. A single pair, or at most two pairs of kidneys are present.

The type of segmentation exhibited by the Crustacea is thus of a limited character, concerning merely the external skin with its appendages, and the nervous system, and not touching any of the other internal organs.[^[1]] In this respect the Crustacea agree with all the other Arthropods, in the adults of which the segmentation is confined to the exterior and to the nervous system, and does not extend to the body-cavity and its contained organs; and for the same reason they differ essentially from all other metamerically segmented animals, e.g. Annelids, in which the segmentation not only affects the exterior and the nervous system, but especially applies to the body-cavity, the musculature, the renal, and often the generative organs. The Crustacea also resemble the other Arthropoda in the fact that the body-cavity contains blood, and is therefore a “haemocoel,” while in the Annelids and Vertebrates the segmented body-cavity is distinct from the vascular system, and constitutes a true “coelom.” To this important distinction, and to its especial application to the Crustacea, we will return, but first we may consider more narrowly the segmentation of the Crustacea and its main types of variation within the group. In order to determine the number of segments which compose any particular Crustacean we have clearly two criteria: first, the rings or somites of which the body is composed, and to each of which a pair of limbs must be originally ascribed; and, second, the nervous ganglia.

Around and behind the region of the mouth there is very little difficulty in determining the segments of the body, if we allow embryology to assist anatomy, but in front of the mouth the matter is not so easy.

In the Crustacea the moot point is whether we consider the paired eyes and first pair of antennae as true appendages belonging to two true segments, or whether they are structures sui generis, not homologous to the other limbs. With regard to the first antennae we are probably safe in assigning them to a true body-segment, since in some of the Entomostraca, e.g. Apus, the nerves which supply them spring, not from the brain as in more highly specialised forms, but from the commissures which pass round the oesophagus to connect the dorsally lying brain to the ventral nerve-cord. The paired eyes are always innervated from the brain, but the brain, or at least part of it, is very probably formed of paired trunk-ganglia which have fused into a common cerebral mass; and the fact that under certain circumstances the stalked eye of Decapods when excised with its peripheral ganglion[^[2]] can regenerate in the form of an antenna, is perhaps evidence that the lateral eyes are borne on what were once a pair of true appendages.

Now, with regard to the segmentation of the body, the Crustacea fall into three categories: the Entomostraca, in which the number of segments is indefinite; the Malacostraca, in which we may count nineteen segments, exclusive of the terminal piece or telson and omitting the lateral eyes; and the Leptostraca, including the single recent genus Nebalia, in which the segmentation of head and thorax agrees exactly with that of the Malacostraca, but in the abdomen there are two additional segments.

It has been usually held that the indefinite number of segments characteristic of the Entomostraca, and especially the indefinitely large number of segments characteristic of such Phyllopods as Apus, preserves the ancestral condition from which the definite number found in the Malacostraca has been derived; but recently it has been clearly pointed out by Professor Carpenter[^[3]] that the number of segments found in the Malacostraca and Leptostraca corresponds with extraordinary exactitude to the number determined as typical in all the other orders of Arthropoda. This remarkable correspondence (it can hardly be coincidence) seems to point to a common Arthropodan plan of segmentation, lying at the very root of the phyletic tree; and if this is so, we are forced to the conclusion that the Malacostraca have retained the primitive type of segmentation in far greater perfection than the Entomostraca, in some of which many segments have been added, e.g. Phyllopoda, while in others segments have been suppressed, e.g. Cladocera, Ostracoda. It may be objected to this view of the primitive condition of segmentation in the Crustacea that the Trilobites, which for various reasons are regarded as related to the ancestral Crustaceans, exhibit an indefinite and often very high number of segments; but, as Professor Carpenter has pointed out, the oldest and most primitive of Trilobites, such as Olenellus, possessed few segments which increase as we pass from Cambrian to Carboniferous genera.

The following table shows the segmentation of the body in the Malacostraca, as compared with that of Limulus (cf. p. 263), Insecta, the primitive Myriapod Scolopendrella, and Peripatus. It will be seen that the correspondence, though not exact, is very close, especially in the first four columns, the number of segments in Peripatus being very variable in the different species.

The appendages of the Crustacea exhibit a wonderful variety of structure, but these variations can be reduced to at most two, and possibly to one fundamental plan. In a typical Crustacean, besides the paired eyes, which may be borne on stalks, possibly homologous to highly modified limbs, there are present, first, two pairs of rod-like or filamentous antennae, which in the adult are usually specialised for sensory purposes, but frequently retain their primitive function as locomotory limbs even in the adult, e.g. Ostracoda; while in the Nauplius larva, found in almost all the chief subdivisions of the Crustacea, the two pairs of antennae invariably aid in locomotion, and the base of the second antennae is usually furnished with sharp biting spines which assist mastication. Following the antennae is a pair of mandibles which are fashioned for biting the food or for piercing the prey, and posterior to these are two pairs of maxillae, biting organs more slightly built than the mandibles, whose function it is to lacerate the food and prepare it for the more drastic action of the mandibles. So far, with comparatively few exceptions, the order of specialisation is invariable; but behind the maxillae the trunk-appendages vary greatly both in structure and function in the different groups.

As a general rule, the first or first few thoracic limbs are turned forwards toward the mouth, and are subsidiary to mastication; they are then called maxillipedes; this happens usually in the Malacostraca, but to a much less extent in the Entomostraca; and in any case these appendages immediately behind the maxillae never depart to any great extent from a limb-like structure, and they may graduate insensibly into the ordinary trunk-appendages. The latter show great diversity in the different Crustacean groups, according as the animals lead a natatory, creeping, or parasitic method of life; they may be foliaceous, as in the Branchiopoda, or biramous, as in the swimming thoracic and abdominal appendages of the Mysidae, or simply uniramous, as in the walking legs of the higher Decapoda, and the clinging legs of various parasitic forms.

Without going into detailed deviations of structure, many of which will be described under the headings of special groups, it is clear from the foregoing description and from Fig. [1] (p. [10]), that three main types of appendage can be distinguished: first, the foliaceous or multiramous; second, the biramous; and, third, the uniramous.

We may dismiss the uniramous type with a few words: it is obviously secondarily derived from the biramous type; this can be proved in detail in nearly every case. Thus, the uniramous second antennae of some adult forms are during the Nauplius stage invariably biramous, a condition which is retained in the adult Cladocera. Similarly the uniramous walking legs of many Decapoda pass through a biramous stage during development, the outer branches or exopodites of the limbs being suppressed subsequently, while the primitively biramous condition of the thoracic limbs is retained in the adults of the Schizopoda, which doubtless own a common ancestry with the Decapoda. The only Crustacean limb which appears to be constantly uniramous both in larval and adult life is the first pair of antennae.

We are reduced, therefore, to two types—the foliaceous and biramous. Sir E. Ray Lankester,[^[6]] in one of his most incisive morphological essays, has explained how these two types are really fundamentally the same. He compares, for instance, the foliaceous first maxillipede (Fig. [1], A), or the second maxilla (Fig. [1], B) of a Decapod, e.g. Astacus, with the foliaceous thoracic limb of Branchipus (Fig. [1], D), and with the typically biramous first maxillipede of a Schizopod (Fig. [1], F).

In each case there is present, on the outer edge of the limb, one or more projections or epipodites which are generally specialised for respiratory purposes, and may carry the gills. The 6th and 5th “endites” in the foliaceous limb (Fig. [1], D) are compared with the exopodite and endopodite respectively of the biramous limb, while the endites 4–1 of the foliaceous limb are found in the basal joints of the biramous limb. Lankester presumes that the biramous type of limb throughout has been derived from the foliaceous type by the suppression of the endites 1–4, as discrete rami, and the exaggerated development of the endites 5 and 6, as above indicated.

Fig. [1].—Appendages of Crustacea (A-G) and Trilobita (H). A, First maxillipede of Astacus; B, second maxilla of Astacus; C, second walking leg of Astacus; D, thoracic limb of Branchipus; E, first maxillipede of Mysis; F, first maxillipede of Gnathophausia; G, thoracic limb of Nebalia; H, thoracic limb of Triarthrus. bp, basipodite; br, bract; cp, carpopodite; cxp, coxopodite; cx.s, coxopoditic setae; dp, dactylopodite; end, endopodite; ep, epipodite; ex, exopodite; ip, ischiopodite; mp, meropodite; pp, propodite; 1–6, the six endites.

The essential fact that the two types of limb are built on the same plan may be considered as established; but it may be urged that the biramous type represents this common plan more nearly than the foliaceous. It is, at any rate, certain that in the maxillipedes of the Decapoda we witness the conversion of the biramous type into the foliaceous by the expansion of the basal joints concomitantly with the assumption by the maxillipedes of masticatory functions. Thus in the Decapoda the first maxillipede is decidedly foliaceous owing to the expanded “gnathobases” (Fig. [1], A, bp, cxp), and the second maxillipedes are flattened, with their basal joints somewhat expanded and furnished with biting hairs; but in the “Schizopoda” (e.g. Mysis) the first maxillipede is a typical biramous limb, though the expanded gnathobases in some forms are beginning to project (Fig. [1], E), while the limb following, which corresponds to the second maxillipede of Decapods, is simply a biramous swimming leg. Besides this obvious conversion of a biramous into a foliaceous limb, further evidence of the fundamental character of the biramous type is found, first, in its invariable occurrence in the Nauplius stage, which does not necessarily mean that the ancestors of the Crustacea possessed this type of limb in the adult, but which does imply that this type of limb was possessed at some period of life by the common ancestral Crustacean; and, second, the limbs of the Trilobita, a group which probably stands near the origin of the Crustacea, have been shown by Beecher to conform to the biramous type (Fig. [1], H). Furthermore, the thoracic limbs of Nebalia, an animal which combines many of the characteristics of Entomostraca and Malacostraca, and is therefore considered as a primitive type, despite their flattened character, are really built upon a biramous plan (Fig. [1], G).

In conclusion, we may point out that this view of the Crustacean limb, as essentially a biramous structure, agrees with the conclusion derived from our consideration of the segmentation of the body, and points less to the Branchiopoda as primitive Crustacea and more to some generalised Malacostracan type.

So far we have shortly dealt with those systems of organs which are clearly affected by the metameric segmentation of the body; we must now expose the condition of the body-cavity to a similar scrutiny. If we remove the external integument of a Crustacean, we find that the internal organs do not lie in a spacious and discrete body-cavity, as is the case in the Annelids and Vertebrates, but that they are packed together in an irregular system of spaces (“haemocoel”) in communication with the vascular system and containing blood. In the Entomostraca and smaller forms generally, a definite vascular system hardly exists, though a central heart and artery may serve to propel the blood through the irregular lacunae of the body-cavity; but in the larger Malacostraca a complicated system of arteries may be present which pour the blood into fairly definitely arranged spaces surrounding the chief organs. These spaces return the blood to the pericardium, and so to the heart again through the apertures or ostia which pierce its walls.

This condition of the body-cavity or haemocoel is reproduced in the adults of all Arthropods, but in some of them by following the development we can trace the steps by which the true coelom is replaced by the haemocoel. In the embryos of all Arthropods except the Crustacea, a true closed metamerically segmented coelom is formed as a split in the mesodermal embryonic layer of cells, distinct from the vascular system. During the course of development the segmented coelomic spaces and their walls give rise to the reproductive organs and to certain renal organs in Peripatus, Myriapoda, and Arachnida (nephridia and coxal glands), but the general body-cavity is formed as an extension of the vascular system, which is laid down outside the coelom by a canaliculisation of the extra-coelomic mesoderm. In the embryos of the Crustacea, however, there is never at any time a closed segmented coelom, and in this respect the Crustacea differ from all other Arthropods. The only clear instance in which metamerically repeated mesodermal cavities have been seen in the embryo Crustacean is that of Astacus; here Reichenbach[^[7]] states that in the abdomen segmental cavities are formed which subsequently break down; but even in this instance no connexion has been shown to subsist between these embryonic cavities and the reproductive and excretory organs of the adult.

Since the connexion between the coelom and the excretory organs is always a very close one throughout the animal kingdom, interest naturally centres upon the renal organs in Crustacea, and it has been suggested that these organs in Crustacea represent the sole remains, with the possible exception of the gonads, of the coelom. Since, at any rate, a part of the kidneys appears to be developed as a closed sac in the mesoderm, and since they possess a possible segmental value, this suggestion is plausible; but, on the other hand, since there are never more than two pairs of kidneys, and since they are totally unconnected with the gonads or with any other indication of a segmented coelom, the suggestion remains purely hypothetical.

The renal organs of the Crustacea, excluding the Malpighian tubes present in some Amphipods which open into the alimentary canal, and resemble the Malpighian tubes of Insects, consist of two pairs—the antennary gland, opening at the base of the second antenna, and the maxillary gland, opening on the second maxilla. These two pairs of glands rarely subsist together in the adult condition, though this is said to be the case in Nebalia and possibly Mysis; the antennary glands are characteristic of adult Malacostraca[^[8]] and the larvae of the Entomostraca, while the maxillary glands (“shell-glands”) are present in adult Entomostraca and larval Malacostraca, that is to say, the one pair replaces the other in the two great subdivisions of the Crustacea. The shell-gland of the Entomostraca is a simple structure consisting of a coiled tube opening to the exterior on the external branch of the second maxilla, and ending blindly in a dilated vesicle, the end-sac. The antennary gland of the Malacostraca is usually more complicated: these complications have been studied especially by Weldon,[^[9]] Allen, and Marchal[^[10]] in the Decapoda. In a number of forms we have a tube opening to the exterior at the base of the second antenna, and expanding within to form a spacious bladder into which the coiled tubular part of the kidney opens, while at the extremity of this coiled portion is the vesicle called the end-sac. This arrangement may be modified; thus in Palaemon Weldon described the two glands as fusing together above and below the oesophagus, the dorsal commissure expanding into a huge sac stretching dorsally down the length of the body. This closed sac with excretory functions thus comes to resemble a coelomic cavity, and the view that it is really coelomic has indeed been upheld.

A modified form of this view is that of Vejdovský, who describes a funnel-apparatus leading from the coiled tube into the end-sac of the antennary gland of Amphipods; he regards the end-sac alone as representing the coelom, while the funnel and coiled tube represent the kidney opening into it.

Not very much is known of the development of these various structures. Some authors have considered that both antennary and maxillary glands are developed in the embryo from ectodermal inpushings, but the more recent observations of Waite[^[11]] on Homarus americanus indicate that the antennary gland at any rate is a composite structure, formed by an ectodermal ingrowth which meets a mesodermal strand, and from the latter are produced the end-sac and perhaps the tubular excretory portions of the gland with their derivatives.

With regard to the possible metameric repetition of the renal organs, it is of interest to note that by feeding Mysis and Nebalia on carmine, excretory glands of a simple character were observed by Metschnikoff situated at the bases of the thoracic limbs.

The alimentary canal of the Crustacea is a straight tube composed of three parts—a mid-gut derived from the endoderm of the embryo, and a fore- and hind-gut formed by ectodermal invaginations in the embryo which push into and fuse with the endodermal canal. The regions of the fore- and hind-gut can be recognised in the adult by the fact of their being lined with the chitinous investment which is continued over the external surface of the body forming the hard exoskeleton, while the mid-gut is naked. The chitinous lining of fore- and hind-gut is shed whenever the animal moults. In the Malacostraca, in which a complicated “gastric mill” may be present, the chitinous lining of this part of the gut is thrown into ridges bearing teeth, and this stomach in the crabs and lobsters reaches a high degree of complication and materially assists the mastication of the food. The gut is furnished with a number of secretory and metabolic glands; the so-called liver, which is probably a hepatopancreas, opening into the anterior end of the mid-gut, is directed forwards in most Entomostraca and backwards in the Malacostraca, in the Decapoda developing into a complicated branching organ which fills a large part of the thorax. In the Decapoda peculiar vermiform caeca of doubtful function are present, a pair of which open into the gut anteriorly where fore-passes into mid-gut, and a single asymmetrically placed caecum opens posteriorly into the alimentary tract where mid- passes into hind-gut.

The disposition of these caeca, marking as they do the morphological position of fore-, mid-, and hind-gut, is of peculiar interest owing to the variations exhibited. From some unpublished drawings of Mr. E. H. Schuster, which he kindly lent me, it appears that in certain Decapods, e.g. Callianassa subterranea, the length of the mid-gut between the anterior and posterior caeca is very long; in Carcinus maenas it is considerable; in Maia squinado it is greatly reduced, the caeca being closely approximated; while in Galathea strigosa the caeca are greatly reduced, and the mid-gut as a separate entity has almost disappeared. The relation of these variations to the habits of the different crabs and to their modes of development is unknown.

The reproductive organs usually make their appearance as a small paired group of mesodermal cells in the thorax comparatively late in life; and neither in their early development nor in the adult condition do they show any clear signs of segmentation or any connexion with a coelomic cavity. The sexes are usually separate, but hermaphroditism occurs sporadically in many forms, and as a normal condition in some parasitic groups (see pp. [105]–107). The adult gonads are generally simple paired tubes, from the walls of which the germ-cells are produced, and as these grow and come to maturity they fill up the cavities of the tubes; special nutrient cells are rarely differentiated, though in some cases (e.g. Cladocera) a few ova nourish themselves by devouring their sister-cells (see p. [44]). The oviducts and vasa deferentia are formed as simple outgrowths from the gonadial tubes, which acquire an opening to the exterior; they are usually poorly supplied with accessory glands, the epithelium of the canals often supplying albuminous secretions for cementing the eggs together, while the lining of the vasa deferentia may be instrumental in the formation of spermatophores for transferring large packets of spermatozoa to the female. In the vast majority of Crustacea copulation takes place, the male passing spermatophores or free spermatozoa into special receptacles (spermathecae), or into the oviducts of the female. The spermatophores are hollow chitinous structures in which the spermatozoa are packed; they are often very large and assume characteristic shapes, especially in the Decapoda.

The spermatozoa show a great variety of structure, but they conform to two chief types—the filiform, which are provided with a long whip-like flagellum; and the amoeboid, which are furnished with radiating pseudopodia, and are much slower in their movements. The amoeboid spermatozoa of some of the Decapoda contain in the cell-body a peculiar chitinous capsule, and Koltzoff[^[12]] has observed that when the spermatozoon has settled upon the surface of the egg the chitinous capsule becomes suddenly exceedingly hygroscopic, swells up, and explodes, driving the head of the spermatozoon into the egg. We cannot enter here into a description of the embryological changes by which the egg is converted into the adult form. Crustacean eggs as a whole contain a large quantity of yolk, but in some forms total segmentation occurs in the early stages, which is converted later into the pyramidal type, i.e. the blastomeres are arranged round the edge, and the yolk in the centre is only partly segmented to correspond with them. The eggs during the early stages of development are in almost all cases (except Branchiura, p. 77, and Anaspides, p. 116) carried about by the female either in a brood-pouch (Branchiopoda, Ostracoda, Cirripedia, Phyllocarida, Peracarida), or agglutinated to the hind legs or some other part of the body (Copepoda, Eucarida), or in a chamber formed from the maxillipedes (Stomatopoda). Development may be direct, without a complicated metamorphosis, or indirect, the larva hatching out in a form totally different to the adult state, and attaining the latter by a series of transformations and moults. The various larval forms will be described under the headings of the several orders.

The respiratory organs are typically branchiae, i.e. branched filamentous or foliaceous processes of the body-surface through which the blood circulates, and is brought into close relation with the oxygen dissolved in the water. In most of the smaller Entomostraca no special branchiae are present, the interchange of gases taking place over the whole body-surface; but in the Malacostraca the gills may reach a high degree of specialisation. They are usually attached to the bases of the thoracic limbs (“podobranchiae”), to the body-wall at the bases of these limbs, often in two series (“arthrobranchiae”), and to the body-wall some way above the limb-articulations (“pleurobranchiae”). In an ideal scheme each thoracic appendage beginning with the first maxillipede would possess a podobranch, two arthrobranchs, and a pleurobranch, but the full complement of gills is never present, various members of the series being suppressed in the various orders, and thus giving rise to “branchial formulae” typical of the different groups.

After this brief survey of Crustacean organisation we may be able to form an opinion upon the position of the Crustacea relative to other Arthropoda, and upon the question debated some time ago in the pages of Natural Science[^[13]] whether the Arthropoda constitute a natural group. The Crustacea plainly agree with all the other Arthropoda in the possession of a rigid exoskeleton segmented into a number of somites, in the possession of jointed appendages metamerically repeated, some of which are modified to act as jaws; they further agree in the general correspondence of the number of segments of which the body is primitively composed; the condition of the body-cavity or haemocoel is also similar in the adult state. An apparently fundamental difference is found in the entire absence during development of a segmented coelom, but since this organ breaks down and is much reduced in all adult Arthropods, it is not difficult to believe that its actual formation in the embryo as a distinct structure might have been secondarily suppressed in Crustacea.

The method of breathing by gills is paralleled by the respiratory structures found in Limulus and Scorpions; the transition, if it occurred, from branchiae to tracheae cannot, it is true, be traced, but the separation of Arthropods into phyletically distinct groups of Tracheata and Branchiata on this single characteristic is inadmissible. On the whole the Crustacea may be considered as Arthropods whose progenitors are to be sought for among the Trilobita, from whose near relations also probably sprang Limulus and the Arachnids.

CHAPTER II
CRUSTACEA (CONTINUED): ENTOMOSTRACA—BRANCHIOPODA—PHYLLOPODA—CLADOCERA—WATER-FLEAS

SUB-CLASS I.—ENTOMOSTRACA.

The Entomostraca are mostly small Crustacea in which the segmentation of the body behind the head is very variable, both in regard to the number of segments and the kind of differentiation exhibited by those segments and their appendages. An unpaired simple eye, known as the Nauplius eye from its universal presence in that larval form, often persists in the adult, and though lateral compound eyes may be present they are rarely borne on movable stalks. In the adult the excretory gland (“shell-gland”) opens on the second maxillary segment, but in the larval state or early stages of development a second antennary gland may also be present, which disappears in the adult. The liver usually points forwards, and is simple and saccular in structure, and the stomach is not complicated by the formation of a gastric mill. With the exception of most Cladocera and Ostracoda the young hatch out in the Nauplius state.

Order I. Branchiopoda.[^[14]]

The Branchiopods are of small or moderate size, with flattened and lobate post-cephalic limbs, and with functional gnathobases. Median and lateral eyes are nearly always present. The labrum is large, and the second maxillae are small or absent in the adult.

Branchiopods are found in every part of the world; a few are marine, but the great majority are confined to inland lakes and ponds, or to slowly-moving streams. The fresh waters, from the smallest pools to the largest lakes, often swarm with them, as do those streams which flow so slowly that the creatures can obtain occasional shelter among vegetation along the sides and bottom without being swept away, while even rivers of considerable swiftness contain some Cladocera. Several Branchiopods are found in the brackish waters of estuaries, and some occur in lakes and pools so salt that no other Crustacea, and few other animals of any kind, can live in them. The great majority swim about with the back downwards, collecting food in the ventral groove between their post-oral limbs, and driving it forwards, towards the mouth, by movements of the gnathobases (p. 10). The food collected in this way consists largely of suspended organic mud, together with Diatoms and other Algae, and Infusoria; the larger kinds, however, are capable of gnawing objects of considerable size, Apus being said to nibble the softer insect larvae, and even tadpoles. Many Cladocera (e.g. Daphnia, Simocephalus) may be seen to sink to the bottom of an aquarium, with the ventral surface downwards, and to collect mud, or even to devour the dead bodies of their fellows, while Leptodora is said to feed upon living Copepods, which it catches by means of its antennae.

The Branchiopoda fall naturally into two Sub-orders, the Phyllopoda including a series of long-bodied forms, with at least ten pairs of post-cephalic limbs, and the Cladocera with shorter bodies and not more than six pairs of post-cephalic limbs.

Sub-Order 1. Phyllopoda.

The Phyllopoda include a series of genera which differ greatly in appearance, owing to differences in the development of the carapace, which are curiously correlated with differences in the position of the eyes. Except in these points, the three families which the sub-order contains are so much alike that they may conveniently be described together.

In the Branchipodidae the carapace is practically absent, being represented only by the slight backward projection on each side of the head which contains the kidney (Fig. [2]); the paired eyes are supported on mobile stalks, and project freely, one on either side of the head.

In the Apodidae[^[15]] the head is broad and depressed, the ventral side being nearly flat, the dorsal surface convex; the hinder margin of the head is indicated dorsally by a transverse cervical ridge, bounded by two grooves, behind which the carapace projects backwards as a great shield, covering at least half the body, but attached only to the back of the head. In Lepidurus productus the head and carapace together form an oval expansion, deeply emarginate at the hinder, narrower end, the sides of the emargination being toothed. The carapace has a strong median keel. The kidneys project into the space between the folds of skin which form the carapace, and their coils can be seen on each side, the terminal part of each kidney-tube entering the head to open at the base of the second maxilla. In all Branchiopoda with a well-developed carapace the kidney is enclosed in it in this way, whence the older anatomists speak of it as the “shell-gland.”

Fig. [2].—Chirocephalus diaphanus, female, × 5, Sussex. D.O, Dorsal organ; H, heart; Ov, ovary; U, uterus; V, external generative opening.

Associated with the development of the carapace, in this and in the next family, is a remarkable condition of the lateral eyes, which are sessile on the dorsal surface of the head, and near the middle line, the median eye being slightly in front of them. During embryonic life a fold of skin grows over all three eyes, so that a chamber is formed over them, which communicates with the exterior by a small pore in front.

In the Limnadiidae the body is laterally compressed, and the carapace is so large that at least the post-cephalic part of the body, and generally the head also, can be enclosed within it.

Fig. [3].—Limnetis brachyura, × 15. (After G. O. Sars.)

In Limnetis (Fig. [3]) the dorsal surface of the head is bent downwards and is much compressed, the carapace being attached to it only for a short distance near the dorsal middle line. The sides of the carapace are bent downwards, and their margins can be pulled together by a transverse adductor muscle, so that the whole structure forms an ovoid or spheroidal case, from which the head projects in front, while the rest of the body is entirely contained within it. When the adductor muscle is relaxed the edges of the carapace gape slightly, like the valves of a Lamellibranch shell, and food-particles are drawn through the opening thus formed into the ventral groove by the movements of the thoracic feet, locomotion being chiefly effected by the rowing action of the second antennae, as in the Cladocera, to which all the Limnadiidae present strong resemblances in their method of locomotion, in the condition of the carapace, and in the form of the telson.

In Limnadia and Estheria the carapace projects not only backwards from the point of attachment to the head, but also forwards, so that the head can be enclosed by it, together with the rest of the body.

In all these genera the carapace is flexible along the middle dorsal line; in Estheria especially the softening of the dorsal cuticle goes so far that a definite hinge-line is formed, and this, together with the deposition of the lateral cuticle in lines concentrically arranged round a projecting umbo, gives the carapace a strong superficial likeness to a Lamellibranch shell, for which it is said to be frequently mistaken by collectors.

The eyes of the Limnadiidae are enclosed in a chamber formed by a growth of skin over them, as in Apodidae, but the pore by which this chamber communicates with the exterior is even more minute than in Apus. The paired eyes are so close together that they may touch (Limnadia, Estheria) or fuse (Limnetis); they are farther back than in the Apodidae, while the ventral curvature of the head causes the median eye to lie below them. In all these points the eyes of the Limnadiidae are intermediate between those of Apus and those of the Cladocera.

Dorsal Organ.—A structure very characteristic of adult Phyllopods is the “dorsal organ” (Figs. [2], 5, D.O), whose function is in many cases obscure. It is always a patch of modified cephalic ectoderm, supplied by a nerve from the anterior ventral lobe of the brain on each side; but its characters, and apparent function, differ in different forms. In the Branchipodidae the dorsal organ is a circular patch, far forward on the surface of the head (Figs. [2], 5, D.O). Its cells are arranged in groups, which remind one of the retinulae in a compound eye; each cell contains a solid concretion, and the concretions of a group may be so placed as to look like a badly-formed rhabdom. Claus,[^[16]] who first called attention to this structure in the Branchipodidae, regarded it as a sense-organ. In Apodidae the dorsal organ is an oval patch of columnar ectoderm, immediately behind the eyes; it is slightly raised above the surrounding skin, and is covered by a very delicate cuticle (with an opening to the exterior?), and below it is a mass of connective tissue permeated by blood; Bernard has suggested that it is an excretory organ.

Most Limnadiidae resemble the Cladocera in the possession of a “dorsal organ” quite distinct from the above; in Limnetis and Estheria it has the form of a small pit, lined by an apparently glandular ectoderm, and this is its condition in many Cladocera; in Limnadia lenticularis it is a patch of glandular epithelium on a raised papilla. Limnadia has been observed to anchor itself to foreign objects by pressing its dorsal organ against them, and many Cladocera do the same thing; Sida crystallina, for example, will remain for hours attached by its dorsal organ to a waterweed or to the side of an aquarium. Structures resembling a dorsal organ occur in the larvae of many other Crustacea, but the presence of this organ in the adult is confined to Branchiopods, and indeed in many Cladocera it disappears before maturity. It is certain that the sensory and adhesive types of dorsal organ are not homologous, especially as rudimentary sense-organs may exist on the head of Cladocera together with the adhesive organ.

The telson differs considerably in the different genera. In the Branchipodidae[^[17]] the anus opens directly backwards; and the telson carries two flattened backwardly directed plates, one on each side of the anus, the margins of each plate being fringed with plumose setae. In Artemia the anal plates are rarely as large as in Branchipus, and never have their margins completely fringed with setae; in A. salina from Western Europe, and in A. fertilis (Fig. [4], A) from the Great Salt Lake of Utah, there is a variable number of setae round the apical half of each lobe, but in specimens of A. salina from Western Siberia the number of setae may be very small, or they may be absent; in the closely allied A. urmiana from Persia the anal lobes are well developed in the male, each lobe bearing a single terminal hair, but they are altogether absent in the female. Schmankewitch and Bateson have shown that there is a certain relation between the salinity of the water in which Artemia salina occurs and the condition of the anal lobes, specimens from denser waters having on the whole fewer setae; the relation is, however, evidently very complex, and further evidence is wanted before any more definite statements can be made.

Fig. [4].—A, Ventral view of the anal region in Artemia fertilis, from the Great Salt Lake; B, ventral view of the telson and neighbouring parts of Lepidurus productus; C, side view of the telson and left anal lobe of Estheria (sp.?).

In the Apodidae the anal lobes have the form of two-jointed cirri, often of considerable length; in Apus the anus is terminal, but in Lepidurus (Fig. [4], B) the dorsal part of the telson is prolonged backwards, so as to form a plate, on the ventral face of which the anus opens, much as in the Malacostraca.

In the Limnadiidae (Fig. [4], C) the telson is laterally compressed and produced, on each side of the anus, into a flattened, upwardly curved process, sharply pointed posteriorly, and often serrate; the anal lobes are represented by two stout curved spines, while in place of the dorsal prolongation of Lepidurus we find two long plumose setae above the anus. In the characters of the telson and anal lobes, as in those of the head, the Limnadiidae approximate to the Cladocera. In Limnetis brachyura the ventral face of the telson is produced into a plate projecting backwards below the anus, in a manner which has no exact parallel among other Crustacea.

The appendages of the Phyllopoda are fairly uniform in character, except those affected by the sexual dimorphism, which is usually great.

Fig. [5].—Chirocephalus diaphanus, male. Side view of head, showing the large second antenna, A₂, with its appendage Ap, above which is seen the filiform first antenna; D.O, dorsal organ; E₁, median eye.

Fig. [6].—Chirocephalus diaphanus. Second antenna of male, uncoiled.

Of the cephalic appendages, the first antennae are generally small, and are never biramous; in Branchipus and its allies they are simple unjointed rods, in some species of Artemia they are three-jointed, in Apus they are feebly divided into two joints, while in Estheria they are many-jointed. The second antennae are the principal organs of locomotion in the Limnadiidae, where they are large and biramous; in all other Phyllopoda they are uniramous in the female, being either unjointed triangular plates as in Chirocephalus (Fig. [2]), or minute vestigial filaments as in Apus, in which genus Zaddach, Huxley, and Claus have all failed to find any trace of a second antenna in some females. In the male Branchipodidae the second antennae are modified to form claspers, by which the female is seized, the various degrees of complication which these claspers exhibit affording convenient generic characters. In Branchinecta each second antenna is a thick, three-jointed rod, the last joint forming a claw, while the second joint is serrate on its inner margin; in Branchipus the base is much thickened, and bears on its inner side a large filament (perhaps represented by the proximal tubercle of Branchinecta and Artemia), which looks like an extra antenna. In Streptocephalus the terminal joint of the antenna is bifid, and there is a basal filament like that of Branchipus; in Chirocephalus diaphanus (Figs. [5], 6) the main branch of the antenna consists of two large joints, the terminal joint being a strong claw with a serrated process at its base, while the proximal joint bears two appendages on its inner side; one of these is a small, subconical tubercle, the second is more complicated, consisting of a main stem and five outgrowths. The main stem is many-jointed and flexible, its basal joint being longer than the others, and bearing on its outer side a large, triangular, membranous appendage, and four soft cylindrical appendages, the main stem and its appendages being beset with curious tubercles, ending in short spines, whose structure is not understood. Except during the act of copulation this remarkable apparatus is coiled on the inner side of the antennary claw, the jointed stem being so coiled that it is often compared to the coiled proboscis of a butterfly, and the triangular membrane folded like a fan beside it, so that much of the organ is concealed, and the general appearance of the head is that shown in Fig. [5]. During copulation, the whole structure is widely extended.

Fig. [7].—Artemia fertilis. Front view of the head of a male, showing the large second antennae, A.2; A.1, first antennae.

The males of Artemia (Fig. [7]) have the second antenna two-jointed, the basal joint bearing an inner tubercle, the terminal joint being flattened and bluntly pointed, its outer margin provided with a membranous outgrowth. In A. fertilis the breadth of the second joint varies greatly, the narrower forms presenting a certain remote resemblance to Branchinecta. In the males of Polyartemia the second antennae have a remarkable branched form not easily comparable with that found in other Branchipodidae.

The cephalic jaws are fairly uniform throughout the order. The mandibles have an undivided molar surface, and no palp; the first maxilla is very generally a triangular plate, with a setose biting edge; mandibles and maxillae are covered by the labrum. The second maxilla generally lies outside the chamber formed by the labrum, and is a simple oval plate, with or without a special process for the duct of the kidney.

The thoracic limbs, in front of the genital segments, are not as a rule differentiated into anterior maxillipedes and posterior locomotive appendages, as in higher forms; we have seen, however, that all these limbs take part in the prehension of food, and except in the Limnadiidae they all assist in locomotion. One of the middle thoracic legs of Artemia (Fig. [8], A) has a flattened stem, with seven processes on its inner, and two on its outer margin. The gnathobase (gn) is large, and fringed with long plumose setae, each of which is jointed; this is followed by four smaller “endites” (or processes on the median side), and then by two larger ones, the terminal endite (the sixth, excluding the gnathobase) being very mobile and attached to the main stem by a definite joint. On the outer side are two processes; a proximal “bract,” a flat plate with crenate edges, partly divided by a constriction into two, and a distal process, cylindrical and vascular, called by Sars and others the “epipodite.” In other Branchipodidae we have essentially the same condition, except that the fifth endite often becomes much larger than in Artemia, throwing the terminal endite well over to the outer edge of the limb; such a shift as this, continued farther, might well lead to the condition found in the Limnadiidae, or Apodidae, where the lobe which seems to represent the terminal endite of Artemia is entirely on the outer border of the limb, forming what most writers have called the exopodite (Lankester’s “flabellum”).[^[18]] In the two last-named families the basal exite or bract of the Branchipodidae does not appear to be represented.

Fig. [8].—A, Thoracic limb of Chirocephalus diaphanus; B, prehensile thoracic limb of male Estheria. gn, Gnathobase; 1–6, the more distal endites.

The limbs of the Apodidae are remarkable in two ways; those in front of the genital opening (very constantly ten pairs) are not so nearly alike as in most genera of the sub-order, the first two pairs especially having the axis definitely jointed, while the endites are elongated and antenniform; further, while the first eleven segments bear each a single pair of limbs, as is usual among Crustacea, many of the post-genital segments bear several pairs; thus in Apus cancriformis there are thirty-two post-cephalic segments in front of the telson, the first eleven having each one pair of limbs, while the next seventeen have fifty-two pairs between them, the last four segments having none.

In all the Phyllopoda some of the post-cephalic limbs are modified for reproductive purposes; in the Branchipodidae the last two pairs (the 12th and 13th generally, the 20th and 21st in Polyartemia) are so modified in both sexes. In the female these appendages fuse at an early period of larval life, and surround the median opening of the generative duct (Fig. [2]); in the male the two pairs also fuse, but traces of the limbs are left as eversible processes round the paired openings of the vasa deferentia.

In the other families, one or more limbs of the female are adapted for carrying or supporting the eggs. In the Apodidae the appendages of the eleventh segment have the exopodite in the form of a rounded, watchglass-shaped plate, fitting over a similarly shaped process of the axis of the limb, so that a lens-shaped box is formed, into which the eggs pass from the oviduct. In Limnadiidae the eggs are carried in masses between the body and the carapace, and are kept in position by special elongations of the exopodites of two or three legs, either those near the middle of the thorax (Estheria, Limnadia), or at its posterior end (Limnetis). In female Limnetis the last thoracic segments bear two remarkable lateral plates, which apparently also help to support the eggs. In the male Limnadiidae, the first (Limnetis) or the first two thoracic feet (Limnadia, Estheria) are prehensile (Fig. [8], B).

Alimentary Canal.—The mouth of the Phyllopoda is overhung by the large labrum, so that a kind of atrium is formed, outside the mouth itself, in which mastication is performed; numerous unicellular glands, opening on the oral face of the labrum, pour their secretion into the atrial chamber, and may be called salivary, though the nature of their secretion is not known. The mouth has commonly two swollen and setose lips, running longitudinally forwards from the bases of the first maxillae, and often wrapping round the blades of the mandibles. It leads into a vertical oesophagus, which opens into a small globular stomach, lying entirely within the head; the terminal part of the oesophagus is slightly invaginated into the stomach, so that a valvular ring is formed at the junction of the two. The stomach opens widely behind into a straight intestine, which runs backwards to about the level of the telson, where it joins a short rectum, leading to the terminal or ventral anus. The stomach and intestine are lined by a columnar epithelium, and covered by a thin network of circularly arranged muscle-fibres; the rectum has a flatter epithelium, and radial muscles pass from it to the body-wall, so that it can be dilated. The only special digestive glands are two branched glandular tubes, situated entirely within the head, which open into the stomach by large ducts, one on each side. In Chirocephalus the gastric glands are fairly small and simple; in the Apodidae their branches are more complex and form a considerable mass, filling all that portion of the head which is not occupied by the nervous system and the muscles. Backwardly directed gastric glands, like those of the higher Crustacea, are not found in Branchiopods; both forms occur together in the genus Nebalia, but with this exception the forwardly-directed glands are peculiar to Branchiopods.

Heart.—In Branchipus and its allies, and in Artemia, the heart extends from the first thoracic segment to the penultimate segment of the body, and is provided with eighteen pairs of lateral openings, one pair in every segment through which it passes except the last; it is widely open at its hinder end, and is prolonged in front for a short distance as a cephalic aorta, the rest of the blood-spaces being lacunar.

In most, at least, of the other Branchiopods, the heart is closed behind and is shortened; in Apus and Lepidurus it only extends through the first eleven post-cephalic segments, while in the Limnadiidae it is shorter still, the heart of Limnetis passing through four segments only. In all cases there is a pair of lateral openings in every segment traversed by the heart.

The blood of the Branchipodidae and Apodidae contains dissolved haemoglobin, the quantity present being so small as to give but a faint colour to the blood in Branchipus, while Artemia has rather more, and the blood of Apus is very red. The only other Crustacea in which the blood contains haemoglobin are the Copepods of the genus Lernanthropus,[^[19]] so that the appearance of this substance is as irregular and inexplicable in Crustacea as in Chaetopods and Molluscs.

The nervous system of Branchipus may be described as an illustration of the condition prevailing in the group. The brain consists of two closely united ganglia, in each of which three main regions may be distinguished; a ventral anterior lobe, a dorsal anterior lobe, and a posterior lobe. The ventral anterior lobes give off nerves to the median eye, to the dorsal organ, and to a pair of curious sense-organs, comparable with the larval sense-knobs of many higher forms, situated one on each side of the median eye; in late larvae Claus describes the terminal apparatus of each frontal sense-organ as a single large hypodermic cell; W. K. Spencer[^[20]] has lately described several terminal cells, containing peculiar chitinous bodies, in the adult. The homologous sense-organs of Limnetis are apparently olfactory. The dorsal anterior lobes give off the large nerves to the lateral eyes, while the posterior lobes supply the first antennae. The oesophageal connectives have a coating of ganglion-cells, and some of these form the ganglion of the second antenna, the nerve to this appendage leaving the connective just behind the brain. The post-oral nerve-cords are widely separate, each of them dilating into a ganglion opposite every appendage, the two ganglia being connected by two transverse commissures. The ganglia of the three cephalic jaws, so often fused in the higher Crustacea, are here perfectly distinct. Closely connected with each thoracic ganglion is a remarkable unicellular gland, opening to the exterior near the middle ventral line; it is conceivable that these cells may be properly compared with the larval nephridia of a Chaetopod,[^[21]] but no evidence in support of such a comparison has yet been adduced.

Behind the genital segments, where there are no limbs, the nerve-cords run backwards without dilating into segmental ganglia, except in the anterior two abdominal segments where small ganglionic enlargements occur. In Apodidae, on the other hand, those segments which carry more than one pair of appendages have as many pairs of ganglia, united by transverse commissures, as they have limbs.

A stomatogastric nervous system exists in Apus, where a nerve arises on each side from the first post-oral commissure, and runs forward to join its fellow of the opposite side on the anterior wall of the oesophagus. From the loop so formed a larger median and a series of smaller lateral nerves pass to the wall of the alimentary canal. A second nerve to the oesophagus is given off from the mandibular ganglion of each side.

Reproductive Organs.—In Chirocephalus the ovaries (Fig. [2], Ov) are hollow epithelial tubes, lying one on each side of the alimentary canal, and extending from the sixth abdominal segment forwards to the level of the genital opening; at this point the two ovaries are continuous with ducts, which bend sharply downwards and open into the single uterus contained within the projecting egg-pouch and opening to the exterior at the apex of that organ. Short diverticula of the walls of the uterus receive the ducts of groups of unicellular glands, the bodies of which contain a peculiar opaque secretion, said to form the eggshells. In Apodidae the ovaries are similar in structure, but they are much larger and branch in a complex manner, while each ovary opens to the exterior independently of the other in the eleventh post-cephalic segment; nothing like the median uterus of the Branchipodidae being formed. The epithelium of the ovarian tubes proliferates, and groups of cells are formed; one becoming an ovum, the others being nutrient cells like those which will be more fully described in the Cladocera.

In Chirocephalus the testes are tubes similar in shape and position to the ovaries, each communicating in front with a short vas deferens, which dilates into a vesicula seminalis on its way to the eversible penis; an essentially similar arrangement is found in all Branchipodidae, but in Apodidae and Limnadiidae there is no penis.

All the Branchiopoda are dioecious,[^[22]] and many are parthenogenetic. Among Branchipodidae Artemia is the only genus known to be parthenogenetic, but parthenogenesis is common in all Apodidae, while the males of several species of Limnadia are still unknown, although the females are sometimes exceedingly common. In Artemia, generations in which the males are about as numerous as the females seem to alternate fairly quickly with others which contain only parthenogenetic females; in Apus males are rarely abundant, and often absent for long periods; during five consecutive years von Siebold failed to discover a male in a locality in Bavaria, though he examined many thousands of individuals; near Breslau he found on one occasion about 11 per cent of males (114 in 1026), but in a subsequent year he found less than 1 per cent; the greatest recorded percentage of males is that observed by Lubbock in 1863, when he found 33 males among 72 individuals taken near Rouen.

The eggs of most genera can resist prolonged periods of desiccation, and indeed it seems necessary for the development of many species that the eggs should be first dried and afterwards placed in water. Many eggs (e.g. of Chirocephalus diaphanus and Branchipus stagnalis) float when placed in water after desiccation, the development taking place at the surface of the water.

Habitat.—All the Phyllopoda, except Artemia, are confined to stagnant shallow waters, especially to such ponds as are formed during spring rains, and dry up during the summer. In waters of this kind the species of Branchipus, Apus, etc., develop rapidly, and produce great numbers of eggs, which are left in the dried mud at the bottom after evaporation of the water, where they remain quiescent until a fresh rainy season. The mud from the beds of such temporary pools often contains large numbers of eggs, which may be carried by wind, on the legs of birds, and by other means, to considerable distances. Many exotic species have been made known to European naturalists by their power of hatching out when mud brought home by travellers is placed in water. The water of stagnant pools quickly dissolves a certain quantity of solid matter from the soil, and often receives dissolved solids through surface drainage from the neighbouring land; such salts may remain as the water evaporates, so that the water which remains after evaporation has proceeded for some time may be very sensibly denser than that in which the Branchiopods were hatched; these creatures must therefore be able to endure a considerable increase in the salinity of the surrounding waters during the course of their lives. My friend Mr. W. W. Fisher points out that the plants present in such a pond would often precipitate the carbonate of lime, so that this might be removed as evaporation went on, but that chlorides would probably remain in solution; from analyses which Mr. Fisher has been kind enough to make for me, it is seen that this happened in a small aquarium in my laboratory, in which Chirocephalus diaphanus lived for four months. In April, mud from the dry bed of a pond, known to contain eggs of Chirocephalus, was placed in this aquarium in Oxford, and water was added from the tap. Oxford tap-water contains about 0·3 grm. salts per litre, the chlorine being equivalent to 0·023 grm. NaCl. Water was added from time to time during May and June, but in July evaporation was allowed to proceed unchecked. At the end of July there was about half the original volume of water, the Chirocephalus being still active; the residue contained 0·96 grm. dissolved solids per litre, with chlorine equal to 0·19 grm. NaCl, so that the percentage of chlorides was about eight times the initial percentage, but there were only three and a fifth times the original amount of total solid matter in solution, the carbonate of lime having precipitated as a visible film.

Some species of Branchipus (e.g. B. spinosus, M. Edw.) and of Estheria (E. macgillivrayi, Baird, E. gubernator, Klutzinger) occur in salt pools, but Artemia flourishes in waters beside whose salinity that endured by any other Branchiopod is insignificant. In the South of Europe, Artemia salina may be found in swarms, as it used to be found in Dorsetshire, in the shallow brine-pans from which salt is commercially prepared; Rathke quotes an analysis showing that a pool in the Crimea contained living Artemia when the salts in solution were 271 grms. per litre, and the water was said to have the colour and consistency of beer.

The behaviour of the animals in the water differs a little; in normal feeding all the species swim with the back downwards, as has already been said; the Branchipodidae rarely settle on the ground, or on foreign objects, but the Apodidae occasionally wriggle along the bottom on their ventral surface, and Estheria burrows in mud.

The greater number of species are found in pools in flat, low-lying regions, and many appear to be especially abundant near the sea; Apus cancriformis has, however, been found in Armenia at 10,000 feet above sea level.

Wells and underground waters do not generally contain Phyllopods; but a species of Branchipus and one of Limnetis, both blind, have been described from the caves of Carniola.

One of the many puzzles presented by these creatures is the erratic way in which they are scattered through the regions they inhabit; a single small pond, a few yards or less in diameter, may be the only place within many miles in which a given species can be found; in this pond it may, however, appear regularly season after season for some time, and then suddenly vanish.

Geographically, the Phyllopoda are cosmopolitan, representatives of every family and of some genera (e.g. Streptocephalus, Lepidurus, Estheria) being found in every one of the great zoological regions, though a few aberrant genera are of limited range, thus Polyartemia is known only from the northern Palaearctic and Nearctic regions, Thamnocephalus only from the Central United States. The genus Artemia is not at present known in Australia.[^[23]] The only recorded British species are Chirocephalus diaphanus, Artemia salina, and Apus cancriformis,[^[24]] but other continental islands, for example the West Indian group, are better supplied. The distribution of the species is very imperfectly known, but on the whole every main zoological region seems to have its own peculiar species, which do not pass beyond its boundaries. Branchinecta paludosa and Lepidurus glacialis are circumpolar, both occurring in Norway, in Lapland, in Greenland, and in Arctic North America; but with these exceptions the Palaearctic and Nearctic species seem to be distinct. The European species Apus cancriformis occurs in Algiers, but the relations between the species of Northern Africa as a whole and those of Southern Europe on the one hand, or of Central and Southern Africa on the other, have yet to be worked out.

The soft-bodied Branchipodidae are not known in the fossil condition;[^[25]] an Apus, closely related to the modern A. cancriformis, has been found in the Trias, but the most numerous remains have been left, as might be expected, by the hard-shelled Limnadiidae; carapaces, closely resembling those of the modern Estheria, are known in beds of all ages from the Devonian period to recent times; these carapaces are in several cases associated with fossils of an apparently marine type. None of the fossil species differ in any important characters from those now living, so that the Phyllopoda have existed in practically their present form for an enormously long period; this fact, and the evidence that species of existing genera were at one time marine, explain the wide distribution of animals at present restricted to a remarkably limited range of environmental conditions.

Summary of the Characters of the Genera.

Sub-Order Phyllopoda.—Branchiopoda with an elongated body, provided with at least ten pairs of post-cephalic limbs, the heart extending through four or more thoracic segments, and having at least four pairs of ostia.

Fam. 1. Branchipodidae.[^[26]]—Carapace rudimentary, eyes stalked; the second antennae flat and unjointed in the female, jointed and prehensile in the male; female generative opening single; telson not laterally compressed, bearing two flattened lobes, or none. The heart extending through the thorax and the greater part of the abdomen.

A. Eleven pairs of praegenital ambulatory limbs.

a. Abdomen of six well-formed segments and a telson; anal lobes well formed, their margins setose.

Branchinecta, Verrill—Second antennae of ♂ without lateral appendages; ovisac of ♀ elongated. B. paludosa, O. F. Müll.—Circumpolar.

Branchiopodopsis, G. O. Sars[^[27]]—Second antennae of ♂ as in Branchinecta; ovisac of ♀ short. B. hodgsoni, G. O. Sars—Cape of Good Hope.

Branchipus, Schaeffer—Second antennae of ♂ with simple internal filamentous appendage. B. stagnalis, Linn.—Central Europe.

Streptocephalus, Baird—Second antennae of ♂ 3–jointed, the last joint bifid; an external filamentous appendage. S. torvicornis, Wagn., Poland.

Chirocephalus, Prévost—Second antennae of ♂ 3–jointed, with a jointed internal appendage, which bears secondary processes, four cylindrical and one lamellar. C. diaphanus, Prévost (Fig. [2], p. 20).—Britain, Central Europe.

b. Abdominal segments five or fewer, and a telson. Anal lobes small or 0, sparsely or not at all setose.

Artemia, Leach—Second antennae of ♂ without filamentous appendage, 2–jointed, the second joint lamellar. A. salina, Linn.—Brine pools of the Palaearctic region.

c. Hinder abdominal segments united with telson to form a fin; anal lobes absent.

Thamnocephalus, Packard—Head with a branched median process of unknown nature. Only species T. platyurus, Packard—Kansas, U.S.A.

B. Nineteen pairs of praegenital ambulatory limbs.

Polyartemia, Fischer—Second antennae of ♂ forcipate; ovisac of ♀ very short. Only species P. forcipata, Fisch.

Fam. 2. Apodidae.[^[28]]—Carapace well developed as a depressed shield, covering at least half the body. Eyes sessile, covered; no male clasping organs; anal lobes long, jointed cirri.

Apus, Scopoli—Telson not produced backwards over the anus; endites of first thoracic limb very long. A. cancriformis, Schaeffer—Britain, Europe, Algiers, Tunis. A. australiensis, Central Australia.

Lepidurus, Leach—Telson produced backwards to form a plate above the anus; endites of first thoracic limb short. L. productus, Bosc.—Central Europe. L. viridis, Southern Australia, New Zealand, L. patagonicus, Bergh, Argentines.

Fam. 3. Limnadiidae.—Body compressed; carapace in the form of a bivalve shell, the two halves capable of adduction by means of a strong transverse muscle; second antennae biramous, alike in both sexes; in the male, the first or the first and second thoracic limbs prehensile; telson laterally compressed.

A. Only the first thoracic limbs prehensile in the male; the carapace spheroidal, without lines of growth; head not included within the carapace-chamber.

Limnetis, Lovén—Compound eyes fused; anal spines absent; ambulatory limbs 10–12. L. brachyura, O. F. Müll (Fig. [3], p. 21).—Norway, Central Europe.

B. The first and second thoracic limbs prehensile in the male; carapace distinctly bivalve, enclosing the head, with concentric lines of growth round a more or less prominent umbo.

Eulimnadia, Packard—Carapace narrowly ovate, with few (4–5) lines of growth. E. mauritani, Guérin—Mauritius. E. texana, Packard—Texas, Kansas.

Limnadia, Brongniart—Carapace broadly ovate, with numerous lines of growth, without distinct umbones; L. lenticularis, Linn.—Northern and Central Europe.

Estheria, Rüppell—Carapace with well-marked umbones and numerous lines of growth, oval; E. tetraceros, Kryneki—Central Europe.

Leptestheria,[^[29]] G. O. Sars—Carapace compressed, oblong. Rostrum with a movable spine; thoracic limbs with accessory lappet on the exopodite. L. siliqua, G. O. Sars—Cape Town.

Cyclestheria,[^[30]] G. O. Sars. C. hislopi, Baird—Queensland, India, East Africa, Brazil.

Sub-Order 2. Cladocera.

The Cladocera are short-bodied Branchiopods, with not more than six pairs of thoracic limbs. The second antennae are important organs of locomotion, and are nearly always biramous; the first antennae are small, at least in the female; the second maxillae are absent in the adult. The carapace may extend backwards so as to enclose the whole post-cephalic portion of the body, or may be reduced to a small dorsal brood-pouch, leaving the body uncovered.

The Cladocera or “Water-fleas” are never of great size; Leptodora hyalina, the largest, is only about 15 mm. long, while many Lynceidae are not more than 0·1 or 0·2 mm. in length.

The head is bent downwards in all the Cladocera, so that parts which are morphologically anterior, such as the median eye and the first antennae, lie ventral to or even behind the compound eyes and the second antennae (cf. Fig. [10]).

The compound lateral eyes fuse at an early period of embryonic life, so that they form a single median mass in the adult, over which a fold of ectoderm grows, to make a chamber over the eye, like that found in the Limnadiidae, except that it is completely closed. The fused eyes are generally large and conspicuous; in some deep-water forms the retinular elements of the dorsal portion are larger than those of the ventral (e.g. Bythotrephes, Fig. [13]). In one or two species which live at very great depths, or in caves, the eyes are altogether absent.

The appendages of the head are fairly uniform, the most variable being the first antennae. In the females of many genera the first antennae are short and immovable, consisting of a single joint, with a terminal bunch of sensory hairs, and often a long lateral hair, as in Simocephalus (Figs. [9], 10), Daphnia, etc. In the female Moina (Fig. [16]) they are movable, as they are in Ceriodaphnia and some others; in Bosmina (Fig. [22]) and many Lyncodaphniidae they are elongated and imperfectly divided into joints by rings of spines, while in Macrothrix they are flattened plates. In the males the first antennae are elongated and mobile (cf. Figs. [11], 19).

Fig. [9].—Simocephalus vetulus, female. Ventral view, without the carapace; A₁, A₂, first and second antennae; For, head; Md, mandible; Te, telson; I-IV, first to fourth thoracic appendages.

The second antennae, the chief organs of locomotion, are biramous in all genera except Holopedium; the number of joints in each ramus, and the number of the long plumose hairs with which they are provided, are remarkably constant in whole series of genera, and are therefore useful for purposes of classification. The creatures row themselves by quick strokes of these appendages, the movement being slow and irregular in the rounder forms, such as Simocephalus or Daphnia, rapid and well directed in such elongated lacustrine forms as Bythotrephes or Leptodora.

The mandibles have no palp; the first maxillae are very small, and the second maxillae are absent (Fig. [9]).

The carapace varies very much. In most genera (the Calyptomera of Sars) it is a large, backwardly-projecting fold of skin, bent downwards at the sides so as to form a bivalve shell, enclosing the whole post-cephalic portion of the body, as in Simocephalus (Fig. [10]). The eggs are laid into the space between the carapace and the dorsal part of the thorax, both the carapace and the thorax itself being often modified for their protection and nutrition. In a few forms, the Gymnomera of Sars, the carapace serves only as a brood-pouch, which is distended when eggs are laid, but collapses to an inconspicuous appendage at the back of the head when it is empty (e.g. Leptodora, Fig. [24], Bythotrephes, Fig. [13]). In the Calyptomera the surface of the carapace is frequently provided with a series of ridges, which may be parallel, rarely branching, as in Simocephalus; or in two sets which cross nearly at right angles, as in Daphnia; or so arranged as to form a hexagonal pattern, as in Ceriodaphnia. In a few forms the whole surface is irregularly covered with spines or scales. The hinder edge of the carapace is often produced into a median dorsal spine (Daphnia, Fig. [19]), or more rarely there are two spines, one at each ventro-lateral corner (Scapholeberis, Fig. [20]).

Fig. [10].—Simocephalus vetulus, × 30. Side view of female, showing the arrangement of the principal organs. A.2, Second antenna; C.S, cervical suture; E, fused compound eyes; H, heart; L, forwardly-directed gastric caeca; N, dorsal organ.

The cuticle of the carapace is often separated from that of the head by a cervical suture, as in Simocephalus (Fig. [10], C.S.) and near the line of demarcation many forms exhibit patches of glandular ectoderm which seem to be homologous with the dorsal adhesive organs of the Limnadiidae. The commonest condition is that of a median dorsal pit (Fig. [10], N.) by means of which the animal can fix itself to foreign objects. Certain forms may remain for long periods of time attached by the dorsal organ to plants, or to the sides of an aquarium, the only movement being a slow vibration of the feet, by which a current of water, sufficiently rapid for respiratory purposes, is established round it.[^[31]] In Sida crystallina (Fig. [11]) the dorsal organ is represented by three structures; in front there is a median raised patch (N.m) of columnar ectoderm, containing concretions like those described in the Branchipodidae, and behind this is a pair of cup-shaped organs (N.e), with raised margins.

Fig. [11].—Sida crystallina, male, × 27. Oxford. A.1, Elongated first antenna; N.e, paired element of dorsal organ; N.m, median element of dorsal organ; Te, testes; ♂, opening of vas deferens.

The fold of skin which forms the carapace contains the coils of the single pair of kidneys, and it forms an important organ of respiration, partly from the great size of the blood-vessels it contains, and partly from the presence of red, blue, or brown respiratory pigments in the tissue of the skin itself.

In most Cladocera the cuticle of the carapace is cast at every ecdysis, with that of other parts of the body; but in Iliocryptus and a few others it remains after each moult, giving the carapace an appearance of “lines of growth,” like that seen in many Limnadiidae.

The segmentation of the body behind the head is obscure, but we can generally recognise (1) a thorax, of as many segments as there are pairs of limbs; (2) an abdomen of three segments; and (3) a telson.

The thoracic limbs of the Calyptomera are flattened, and resemble those of the Phyllopoda; as a type we may examine the third thoracic limb of Simocephalus (Fig. [12], C), in which the axis bears a large setose gnathobase (Gn) on its inner edge, followed by two small endites; the terminal process, or exopodite (Ex) is a large flattened plate, with six long plumose hairs on its edge. The outer margin of the axis bears a bract (Br) and an epipodite.

In Simocephalus, as in the other Daphniidae, there are five pairs of thoracic limbs, of which the third and fourth are alike; in the female each limb of the first pair consists of a jointed axis, with strong biting hairs on the inner border, and a rudimentary epipodite (Fig. [12], A), the second limb being more like the third, but with a more prominent gnathobase and a narrower exopodite (B), while the limbs of the fifth pair have the gnathobase and the exopodite filamentous (D).

In the Sididae there are six pairs of thoracic limbs, which are nearly alike in the female; in the Bosminidae there are six pairs, the first two modified for prehension, the last much reduced.

Fig. [12].—Thoracic limbs of female Simocephalus vetulus. A, The first; B, the second; C, the third; D, the fifth. Br, Bract; Ep, epipodite; Ex, exopodite; Gn, gnathobase.

In the male, the first thoracic limb is usually provided with a long sensory process and a prehensible hook (Figs. [11], 19).

In the Gymnomera the limbs are cylindrical, jointed rods, with a gnathobase on the inner side in the Polyphemidae, but not in Leptodora. The number varies from four to six pairs.

The abdomen bears no appendages. The telson is compressed in the Calyptomera, and is produced into two flattened plates, one on each side of the anal opening. The backwardly directed margins of these plates are commonly serrated, and the lower corner of each is produced into a curved spine, which carries secondary teeth. The number and arrangement of these teeth, though often extremely variable in the same species, are used extensively as specific characters. Above the anus the telson commonly bears two long plumose hairs, which are directed backwards.

Fig. [13].—Bythotrephes cederströmii, female, × 20, North Wales, from a specimen found by A. D. Darbishire. Car, carapace.

In the Gymnomera the telson is not bilaterally compressed, and it may be produced into a long spine, dorsal to the anus (e.g. Bythotrephes, Fig. [13]).

The alimentary canal is extremely simple. The labrum is large, and forms a chamber above the mouth, into which food is driven by the limbs, as in the Phyllopoda, food being taken while the animal swims or lies on its back. The oesophagus runs vertically to join a small stomach, which bends sharply backwards and passes gradually into an intestine. In the last segment of the abdomen the intestine joins a short, thin-walled rectum, provided with radial muscles, by means of which it can be dilated. The dilatation of the rectum leads to an inhalation of water through the anus, which may possibly serve as a means of respiration. In the Daphniidae and Bosminidae there are two forwardly-directed digestive glands which open into the stomach, and in Eurycercus there is a large caecum at the junction of the rectum with the intestine. The intestine is usually straight, but in Lynceidae and in some Lyncodaphniidae it is coiled (e.g. Peracantha, Fig. [14]).

In Leptodora the alimentary canal is altogether remarkable; the oesophagus is a long and very narrow tube, which runs back through the whole length of the thorax and joins the mid-gut in the third abdominal segment. The mid-gut is not differentiated into stomach and intestine; it has no diverticula of any kind, and runs straight backwards to join the short rectum a little in front of the anus.

Fig. [14].—Peracantha truncata, female, × 100. Oxford.

The heart is always short, and never has more than a single pair of lateral openings; it is longest in the Sididae, which show some approximation to the Phyllopods in this, as in the slight degree of difference between their anterior and posterior thoracic limbs. The pericardium lies in the one or two anterior thoracic segments, dorsal to the gut. From the heart the blood runs forwards to the dorsal part of the head, and passes backwards by three main channels, one entering each side of the carapace, while the third runs down the body, beneath the alimentary canal to dilate into a large sinus round the rectum. This ventral blood-channel gives a branch to each limb, which forms a considerable dilatation in the epipodite, the blood from the limb returning to the pericardium by a lateral sinus. From the rectum a large sinus runs forwards to the pericardium along the dorsal wall of the body. The blood which enters each half of the carapace is collected in a median vessel and returned through this to the pericardium.

Those spaces between the viscera which are not filled with blood are occupied by a peculiar connective tissue, consisting of rounded or polyhedral cells, charged with drops of a fatty material which is often brightly coloured.

The reproductive organs are interesting because of the peculiar phenomena connected with the nutrition of the two kinds of eggs. The ovaries or testes are epithelial sacs, one on each side of the body, each continuous with a duct which opens to the exterior behind the last thoracic limb. In the female, the opening is dorsal (Fig. [10]), in the male it is ventral (Fig. [11]). The external opening is usually simple; but in the male there is sometimes a penis-like process, on which the vas deferens opens (Daphnella).

The eggs are of two kinds, the so-called “summer-eggs,” with relatively little yolk, which develop rapidly without fertilisation, and the so-called “winter-eggs,” containing much yolk, which require to be fertilised and then develop slowly.

At one end of the ovary, generally that nearest to the oviduct, there is a mass of protoplasm, containing nuclei which actively divide; this is the germarium (Fig. [15], A, B, C). As a result of proliferation in the germarium, nucleated masses are thrown off into the cavity of the ovary; each such mass contains four nuclei, and its protoplasm soon becomes divided into four portions, one round each nucleus, so that four cells are produced. In the simpler ovaries, such as that of Leptodora (Fig. [15], A), these sets of four cells are arranged in a linear series within the tube of ovarian epithelium; in other cases, as in Daphnia, the arrangement is more irregular. In the normal development of parthenogenetic eggs, one cell out of each set of four becomes an ovum, the other three feeding it with yolk and then dying. Weismann[^[32]] has shown that the ovum is always formed from the third cell of each set, counting from the germarial end, so that in the ovary of Leptodora drawn in Fig. [15], A, the ova will be formed from the cells marked E₁, E₂, E₃. At certain times, one or two sets of germinal cells fail to produce ova; the epithelial wall of the ovary thickens round these cells, so that they become incompletely separated from the rest in a so-called “nutrient chamber” (Fig. [15], B, N.C). Germ-cells enclosed in a nutrient chamber degenerate and are ultimately devoured by the ovarian epithelium. The significance of these nutrient chambers is unknown.

Fig. [15].—A, Ovary of a parthenogenetic Leptodora hyalina; B, base of another ovary of the same species, showing a so-called “nutrient chamber”; C, ovary of a female Daphnia, showing the formation of a winter-egg. E, E₁–E₃, Parthenogenetic egg; Ep, ovarian epithelium; G, germarium; N.C, nutrient chamber; O.D, oviduct; W, winter-egg; 1, 2, 4, the other three cells of the same group; II, III, two other groups of germ-cells.

The production of a winter-egg is a more complicated process. The epithelium of the ovarian tube swells up, so that the lumen is nearly obliterated, and several sets of four germ-cells pass from the germarium to lie among the swollen epithelial cells. All these groups of germ-cells, except one, disintegrate and are devoured by the ovarian epithelium, one cell of the remaining group enlarging to form a winter-egg, fed during its growth not only by the three cells of its own set but also by the epithelial cells of the ovarian tube, which have devoured the germ-cells of other sets. An ovary never contains more than a single winter-egg at the same time, the number of germ-cells which are devoured during its formation varying in the different species; the Daphnia drawn in Fig. [15], C, has produced three groups of germ-cells, of which two (II, III), will die, while the cell W from the remaining group will develop into an ovum; in Moina, Weismann finds that as many as a dozen cell-groups may be thrown into the ovary before the production of a winter-egg, so that only one out of forty-eight germ-cells survives as an ovum.

Fig. [16].—Sketch of a parthenogenetic Moina rectirostris, × 45, the brood-pouch being emptied and the side of the carapace removed, showing the dome of thickened epithelium on the thorax, by which nutrient material is thrown into the brood-pouch, and the ridge which fits against the carapace in the natural condition so as to close the brood-pouch.

Fig. [17].—Moina rectirostris, ♀, × 40, showing the ephippial thickening of the carapace which precedes the laying of a winter-egg.

The summer-eggs are always carried until they are hatched by the parthenogenetic female which produces them. The brood-pouch is the space between the dorsal wall of the thorax and the carapace. This space is always more or less perfectly closed at the sides by the pressure of the carapace against the body, and behind by vascular processes from the abdominal segments (Figs. [10], 16, etc.). The presence of a large blood-sinus beneath the dorsal wall of the thorax and in the middle line of the carapace suggests the possibility that some special nutrient substances may pass from the body of the parent into the brood-chamber, and in some species the thoracic ectoderm is specially modified as a placenta. In Moina (Fig. [16]) the dorsal wall of the thorax is produced into a dome, covered by a columnar ectoderm, which contains a dilatation of the dorsal blood-sinus; and in this form it has been shown that the fluid in the brood-pouch contains dissolved proteids. Associated with the apparatus for supplying the brood-pouch with nutriment is a special apparatus for closing it, in the form of a raised ridge, which projects from the back and sides of the thorax and fits into a groove of the carapace.

A somewhat similar nutrient apparatus exists in the Polyphemidae, where the edges of the small carapace are fused with the thorax, so that the brood-pouch is completely closed, and the young can only escape when the parent casts her cuticle. In some genera of this family (e.g. Evadne) the young remain in the parental brood-pouch until they are themselves mature, so that when they are set free they may already bear parthenogenetic embryos in their own brood-pouches.

Fig. [18].—Newly-cast ephippium of Daphnia, containing two winter-eggs.

The winter-eggs are fertilised in the same part of the carapace of the female in which the parthenogenetic eggs develop, but after fertilisation they are thrown off from the body of the mother, either with or without a protective envelope formed from the cuticle of the carapace. The eggs of Sida are surrounded by a thin layer of a sticky substance, and when cast out of the maternal carapace they adhere to foreign objects, such as water-weeds; those of Polyphemus have a thick, gelatinous coat; in Leptodora and Bythotrephes the egg secretes a two-layered chitinous shell. In these forms the cuticle of the parent is not used as a protection for the winter-eggs, although it is generally, if not invariably, thrown off when the eggs are laid. In the Lynceidae the cuticle is moulted in such a way that the winter-eggs remain within it, at least for a time; the cuticle is occasionally modified before it is thrown off; thus in Camptocercus macrurus the cuticle of the carapace, in the region of the brood-pouch, becomes thickened and darkly coloured, forming a fairly strong case round the eggs. The modification of the cuticle round the brood-pouch is much more pronounced in the Daphniidae, where it leads to the formation of a saddle-shaped cuticular box, the “ephippium,” in which the winter-eggs are enclosed. The ripening of a winter-egg in the ovary of a Daphnia is accompanied by a great thickening of the cuticle of the carapace (cf. Fig. [18]), so that a strong case is formed in the position of the brood-pouch. The winter-eggs are laid between the two valves of this case, and shortly afterwards the parent moults. The eggs are retained within the ephippium, from which the rest of the cuticle breaks away (Fig. [18]). After separation, the ephippium, which contains a single egg (Moina rectirostris) or usually two (Daphnia, etc.), either sinks to the bottom, as in Moina, or floats.

The winter-eggs usually go through the early stages of segmentation within a short time after they are laid, but after this a longer or shorter period of quiescence occurs, during which the eggs may be dried or frozen without injury. The sides and floor of a dried-up pond are often crowded with ephippia, containing winter-eggs which develop quickly when replaced in water; and the resting-stage of winter-eggs produced in aquaria can often be materially shortened by drying the ephippia which contain them, though such desiccation does not appear to be necessary for development. Under normal conditions large numbers of winter-eggs remain quiescent through the winter and hatch in the following spring.

The individual developed from a sexually fertilised winter-egg is invariably a parthenogenetic female: the characters of the succeeding generations differ in different cases.

In a few forms, of which Moina is the best known, the parthenogenetic female, produced from a winter-egg, may give rise to males, to sexual females, and to parthenogenetic females, so that the cycle of forms which intervene between one winter-egg and the next is short. A sexual female produces one or two winter-eggs, and if these are fertilised they are enclosed in an ephippium and cast off; if, however, the eggs when ripe are not fertilised, they atrophy, and the female produces parthenogenetic eggs, being thenceforward incapable of forming sexual “winter” eggs. An accidental absence of males may thus lead to the occurrence of parthenogenesis in the whole of the second generation. The regular production of sexual individuals in the second generation from the winter-egg appears to depend on a variety of circumstances not yet understood. Mr. G. H. Grosvenor tells me that Moina from the neighbourhood of Oxford may give rise to several successive generations of parthenogenetic individuals, when grown in small aquaria.

In the greater number of Daphniidae, the parthenogenetic female, produced from a winter-egg, gives rise only to parthenogenetic forms, and it is not until after half a dozen parthenogenetic generations have been produced that a few sexual forms appear, mixed with the others. Such sexual forms are fairly common in April or May in this country; they produce “winter” eggs and then die, the generations which succeed them through the summer being entirely parthenogenetic. In late autumn sexual individuals are again produced, giving rise to a plentiful crop of winter-eggs, but many parthenogenetic females are still found, and some of these appear to live and to reproduce through the winter.

In Sida, in the Polyphemidae and Leptodoridae, and in most of the Lynceidae, sexual individuals are produced only once in every year, while in a few forms which inhabit great lakes the sexual condition occurs so rarely that it is still unknown.

Weismann[^[33]] has pointed out that the sexual forms, with their property of producing eggs which can endure desiccation, recur most frequently in species such as Moina, which inhabit small pools liable to be dried up at frequent intervals, while the species which produce sexual forms only once a year are all inhabitants either of great lakes which are never dry, or of the sea. Many suggestions have been made as to the environmental stimulus which induces the production of sexual individuals, but nothing is definitely known upon the subject.

We have said that even in those generations which contain sexual males and females there are always some parthenogenetic individuals; there is therefore nothing in the behaviour of Daphniidae, either under natural conditions or when observed in aquaria, to suggest that there is any natural or necessary limit to the number of generations which may be parthenogenetically produced.

The parthenogenetic Daphniidae are extremely sensitive to changes in their surroundings; small variations in the character and amount of substances dissolved in the water are often followed by changes in the length of the posterior spine, in the shape and size of crests on the head, and in other characters affecting the appearance of the creatures, so that the determination of species is often a matter of great difficulty. It is remarkable that the green light which has passed through the leaves of water-plants appears to have a prejudicial effect upon some species. Warren has shown that Daphnia magna reproduces more slowly when exposed to green light, and that individuals grown in this way are more readily susceptible to injury from the presence of small quantities of salt (sodium chloride) in the water than individuals which have been exposed to white light.

The majority of the Cladocera belong to the floating fauna of the fresh waters and seas; a few are littoral in their habits, clinging to water-weeds near the shore, a very few live near the bottom at considerable depths, but the majority belong to that floating fauna to which Haeckel gave the name of “plankton.” The Crustacea are an important element in the plankton, whether in fresh waters or in the sea, the two great groups which contribute most largely to it being the Cladocera and the Copepoda. For this reason it will be more convenient to discuss the habits and distribution of individual Cladocera and Copepoda together in a chapter specially devoted to the characters of pelagic faunas (cf. Chap. VII.). We will only add to the present chapter a table of the families with a diagnosis of the British genera.

Summary of Characters of the British Genera.[^[34]]

Tribe I. Calyptomera, Sars.—The post-cephalic portion of the body enveloped in a free fold or carapace.

A. Six pairs of thoracic feet, the first pair not prehensile (Ctenopoda).

Fam. 1. Sididae: second antennae biramous in both sexes. Sida, Straus (Fig. [11]): second antenna with three joints in the dorsal ramus, two in the ventral; the rostrum large, the teeth on the telson many. Latona, Straus: second antenna with two joints in the dorsal ramus, three in the ventral, the proximal joint of the dorsal ramus provided with a setose appendage. Daphnella, Baird: second antenna with the joints as in Latona, but with no setose appendage.

Fam. 2. Holopediidae: second antennae not biramous in the female; a rudimentary second ramus in the male. Holopedium, Zaddach.

B. Four to five or six pairs of thoracic feet, the anterior pair prehensile (Anomopoda).

A. Ventral ramus of second antenna with three joints, the dorsal ramus with four.

Fam. 3. Daphniidae: five pairs of thoracic feet, with a gap between the fourth and fifth pairs. The stomach with two forwardly-directed diverticula.

Fig. [19].—Daphnia obtusa, male, × about 50. Oxford. A.1, First antenna; Th.1, first thoracic appendage.