CHAPTER I. Crustacea—General Organisation

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. 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 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 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.

TABLE SHOWING THE SEGMENTATION OF VARIOUS ARTHROPODS ┌──┬──────────────┬───────────┬────────────┬─────────────┬────────────┐ │ │Malacostraca. │Limulus. │ Insecta. │ Myriapoda. │Peripatus.│ │ │ │ │ │ (Scolopen- │ │ │ │ │ │ │ drella). │ │ ├──┼──────────────┼───────────┼────────────┼─────────────┼────────────┤ │ 1│Eyes │Median eyes│Eyes │ │ │ │ 2│1st antennae │Rostrum │Antennae │Feelers │Feelers │ │ 3│2nd antennae │Chelicerae │Intercalary │ │ │ │ │ │ │segment │ │ │ │ 4│Mandibles │Pedipalpi │Mandibles │Mandibles │Mandibles │ │ 5│1st maxillae │1st walking│Maxillulae │Maxillulae │1st jaw-claw│ │ │ │legs │ │ │ │ │ 6│2nd maxillae │2nd „ │1st maxillae│1st maxillae │2nd jaw-claw│ │ │ │ „ │ │ │ │ │ 7│1st │3rd „ │2nd maxillae│2nd maxillae │1st leg │ │ │maxillipede │ „ │ │ │ │ │ 8│2nd │4th „ │1st leg │1st leg │2nd „ │ │ │maxillipede │ „ │ │ │ │ │ 9│3rd │Chilaria │2nd „ │2nd „ │3rd „ │ │ │maxillipede │ │ │ │ │ │10│1st ambulatory│Genital │3rd „ │3rd „ │4th „ │ │ │ │operculum │ │ │ │ │11│2nd „ │1st │1st │4th „ │5th „ │ │ │ │gill-book │abdominal │ │ │ │12│3rd „ │2nd „ │2nd „ │6th „ │6th „ │ │13│4th „ │3rd „ │3rd „ │6th „ │7th „ │ │14│5th „ │4th „ │4th „ │7th „ │8th „ │ │15│1st abdominal │5th „ │5th „ │8th „ │9th „ │ │16│2nd „ │No │6th „ │9th „ │10th „ │ │ │ │appendages │ │ │ │ │17│3rd „ │ „ │7th „ │10th „ │11th „ │ │18│4th „ │ „ │8th „ │11th „ │12th „ │ │19│5th „ │ „ │9th „ │12th „ │13th „ │ │20│6th „ │ „ │10th „ │Reduced limbs│14th „ │ │21│ │ „ │ │Cercopods │ │ │ │Telson │Telson │Telson │Telson │Telson │ └──┴──────────────┴───────────┴────────────┴─────────────┴────────────┘

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, 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 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 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, Allen, and Marchal 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 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 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 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.