Crustacea (part 2)

The constant association of a large group of animals like the Hermit-crabs with the appropriated empty houses of another group is sufficiently curious, but it does not stop there. In almost every case there are present one or more Sea-anemones growing on the outside of the shell, and each kind of Hermit-crab generally carries a special kind of Anemone. Thus at Plymouth, Eupagurus bernhardus is generally symbiotic with Sagartia parasitica, or else with a colony of Hydractinia echinata, while E. prideauxii is usually associated with Adamsia palliata. In the latter case the shell is frequently absorbed, so that the Anemone comes to envelop the crab like a blanket. Instead of Anemones carried turret-like and imposing aloft, or enveloping the inmate of the shell like a blanket, some of the Hermits have Sponges, an unexpected association; and it is a common sight at Naples to find the little red round Sponge, Suberites, running around animated by its Hermit within. It is held that Anemone and crab mutually assist one another, that the Anemone stings the crab’s enemies, and that the Hermit-crab carries the Anemone to new feeding-grounds. It is also said that when a crab grows too big for its shell, and is forced to seek another, it persuades the Anemone to loosen its attachment to the deserted shell and to be transplanted to the new one, and that there is something mesmeric in its power, because nobody else can pull an Anemone off a shell without either cutting it off at the base or tearing it to pieces. Other animals as well sometimes enter into this partnership. At Plymouth a Polychaet worm, Nereis fucata, frequently inhabits the Whelk’s shell, together with Eupagurus bernhardus, and puts out its head for a share of each meal; and at Naples the Amphipod Lysianax punctatus is almost always present in the shells of Eupagurus prideauxii.

Besides the ordinary twisted Pagurids which inhabit Gasteropod shells, there are a few which preserve the symmetry of the body. The interesting Pylocheles miersii (Fig. ), taken by the Investigator in the Andaman Sea at 185 fathoms, inhabits pieces of bamboo; it is perfectly symmetrical, with well-developed pleopods and symmetrical chelae, which, when the animal is withdrawn, completely shut up the entrance to its house (Fig. , A).

It is doubtful whether this animal ever inhabited a spiral shell or not in its past history; but there is no doubt that a number of peculiar crabs, which caused the older systematists much trouble, are Pagurids, derived from asymmetrical shell-haunting ancestors that have secondarily taken to a different mode of life, and lost, or partially lost those characteristics of ordinary Hermit-crabs which are associated with life in a spiral shell. These are the Lithodidae and the “Robber-crab,” Birgus latro, of tropical coral islands.

Although the Robber-crab and the Lithodidae bear a certain superficial resemblance to one another in that they lead a free existence, and have reacquired to a great extent their symmetry, yet it is clear that they have been independently derived from different groups of asymmetrical Hermit-crabs, and that their resemblance to one another is due to convergence.

Birgus latro (Fig. ), a gigantic crab, frequently over a foot in length, lives on land, and inhabits the coasts of coral islands in the Indian and Pacific Oceans where cocoa-nut trees grow. It feeds on the pulp of the cocoa-nut, which it extracts by hammering with its heavy chela on the “eye-hole” until room is made for the small chela to enter and extract the pulp. There is not the slightest doubt that the animal often ascends the cocoa-nut trees for the purpose of picking the nuts, a fact illustrated by a fine photograph by Dr. Andrews, exhibited in the Crustacean Gallery in the Natural History Departments of the British Museum. It uses the husk of the nut to line its burrow, and it is said to have the habit of putting its abdomen into the nut-shell for protection and carrying it about with it. Owing to its terrestrial mode of life, the branchial chamber is highly modified, being divided into two portions—a dorsal space, the lining of which is thrown into vascular ridges and folds for aerial respiration, and a lower portion where the rudimentary branchiae are situated. Although the Robber-crab lives ordinarily on land, it must be supposed that these branchiae are of some service; the young are hatched out as ordinary Zoaeas in the sea, and go through a pelagic existence before seeking the land. At the present time the Robber-crab is confined to the Pacific and the islands of the Indian Ocean, wherever the cocoa-nut grows. It seems, however, that its association with the cocoa-nut is a comparatively modern one. Mr. C. Hedley, of Sydney, who has had great experience of the Pacific Islands, informs me that the cocoa-nut is not, as is usually supposed, a native of these coral islands, but has been introduced, probably from Mexico, by the Polynesian mariners before the discovery of America by Columbus. Before the introduction of the cocoa-nut the Robber-crab must have fed on some other tree, possibly the Screw Pine, Pandanus.

The abdomen is full of oil, and is much prized as a delicacy by the natives, who tell many strange legends about the creature, but the philosopher may well find its structure more strange than fiction, and the consideration of its morphology an intellectual feast.

The appearance of the thorax and of the thoracic limbs is thoroughly Pagurid; the structure of the abdomen is highly peculiar.

From the ventral surface (Fig. ) we can see at the tip of the tail three small calcified plates, which represent the fifth and sixth terga and the telson. Attached to the sixth segment are the much reduced and rudimentary pleopods of that segment, and on the left hand side of the body in the female are three well-developed pleopods of the first, second, and third segments, which are used for carrying the eggs. The extraordinary asymmetry of these limbs compared with the complete symmetry of the abdomen itself is only explicable on the hypothesis that these animals are descended from Hermit-crabs which had lost the pleopods on the right side.

These appendages are entirely absent in the male. The ventral surface of the abdomen is curiously warty and rugose, and is very soft and pulpy owing to the immense store of oil which it contains.

If we look at the dorsal surface of the abdomen we find that, unlike that of the Hermit-crabs, it is completely protected by a number of hard plates (Fig. , B). Beneath the carapace can be seen a number of small plates belonging to the last thoracic segment; following these there are four large plates (1–4) representing the terga of the first four abdominal segments; the fifth, sixth, and the telson are, as has been stated, carried on the under side of the abdomen, but they are represented diagrammatically (5, 6, T) in the dorsal view. Besides the large terga, there are a number of small plates laterally, usually two to each segment, but they show a tendency to subdivide and increase in the largest specimens. This condition of affairs is very different to that in the naked fleshy abdomen of an ordinary Pagurid, but it can easily be deduced from that of the genus Cenobita, ordinary Hermit-crabs found in the Indo-Pacific Oceans, from which the Robber-crab has evidently descended. In Cenobita (Fig. , A) we see the same system of plates upon the dorsal surface of the abdomen, but they are much smaller, and the lateral plates are not so numerous; indeed, the greater part of the abdomen remains fleshy and uncalcified. The under surface of the abdomen shows the same rugosity as is found in Birgus, and from a number of other anatomical characters it is evident that the Robber-crab is a highly modified Cenobita that has deserted its shell and developed a symmetrical abdomen protected by expanded and hardened plates which represent those found in a reduced condition in Cenobita. The species of Cenobita although they inhabit shells and have normal branchiae, live on the shore, and have not been seen to descend actually into the sea.

The Lithodidae, which are found in temperate seas, especially on the Northern Pacific coasts (though Lithodes maia occurs in the North Sea, and certain species inhabit deep water in the Indian Ocean), have a deceptively Brachyuran appearance, the thorax being much shortened and the abdomen being much reduced and carried tightly flexed on to the ventral surface of the thorax. They live a free, unprotected existence, and are highly calcified. They are, however, certainly Pagurids, as is evidenced by a number of anatomical characters, but most clearly by the asymmetry of the abdomen, especially in the female, which is not only markedly asymmetrical in the arrangement of its dorsal plates (Fig. ), but also in the presence of three pleopods upon the left side only, as in Birgus. The male is without these appendages, and the sixth pair of pleopods is absent in both sexes. The remarkable calcified plates upon the abdomen bear a superficial resemblance to those in Birgus, but their evolution is traced, not from a Cenobite, but from an Eupagurine stock.

In some of the Eupagurinae, e.g. Pylopagurus, feebly calcified plates are present upon the segments of the abdomen (Fig. , A).

In the most primitive of the Lithodidae we witness the reduction (Fig. , B) and disappearance (C) of these original plates, their place being taken first by a number of irregularly situated small spines and warts, which, however, subsequently fuse up to form definite segmental plates. In Lithodes maia, ♂ (D), there are a series of lateral and marginal plates, while in Acantholithus (E) a number of median plates appear, presumably by the fusion of the small spines present in the median line in Lithodes maia; finally, a fusion of the marginal and lateral plates may take place, so that each abdominal segment is covered by a median and two paired lateral plates.

It is to be noted that the males and females of the various species do not follow a parallel course of development, the plates in the male being symmetrical, while those of the female are often highly asymmetrical (compare Figs. , D, and 121), thus giving the strongest evidence of a Pagurid ancestry.

Birgus and the Lithodidae, then, are Pagurids which have given up living in shells, and have become adapted to a free existence, protecting their soft parts by the development of hard plates, and re-acquiring, to a greater or less degree, a secondary symmetry of form. But the story of Pagurid evolution does not apparently stop here. The genus Paralomis, from the West Coast of America, superficially resembles Porcellana, and is held to be descended from such forms as Pylocheles, while isolated species are known (though not well known), such as Tylaspis, described in the Challenger Reports, which appear to be Pagurids that have deserted their shells.

The metamorphosis of the Hermit-crabs has recently been studied by M. T. Thompson.

The Zoaea (Fig. , A) differs from that of the Galatheidea mainly in the absence of the long spines. It possesses the usual appendages characteristic of the Zoaea, namely, the first and second antennae, mandibles, first and second maxillae, and two pairs of biramous swimming maxillipedes and small third maxillipedes. In the Metazoaea (B), as in the Anomura generally, the third maxillipedes develop into biramous swimming organs, a thing they never do in the Brachyura, and the rudiments of the thoracic segments put in a first appearance. The abdominal segments are already fully formed in the Zoaea stage, so that here as in all other Zoaeas, the order of development from in front backwards is disturbed by the precocious differentiation of the abdominal segments. The next stage is the “Glaucothoe” (Fig. , C), which corresponds to the Megalopa of Brachyura (Fig. , p. 183). It differs from the adult Hermit-crab in the perfect symmetry of its body, the segmented abdomen, and the presence of five pairs of normal biramous pleopods. At this stage, which lasts four or five days, it resembles closely a little Galatheid. The asymmetry of the adult (Fig. , D) is now imposed upon this larva by the migration of the liver, gonads, and green glands into the abdomen, and by the shifting of the posterior lobes of the liver on to the left side of the intestine, which is displaced dorsally and to the right. The gonad lies entirely on the left side. The pleopods of the right side now degenerate, more completely in the male than in the female, and this degeneration is not completed until the little crab has found a shell and lived in it for some time. If a shell is withheld from it, the degeneration of the pleopods is much retarded, so that although the Hermit-crab assumes its asymmetry without the stimulus of the spiral shell, yet this stimulus is necessary for the normal completion of the later stages.

Fam. 1. Pylochelidae.—The abdomen is macrurous and symmetrical, with all the limbs present. Pylocheles (Fig. , p. 173).

Fam. 2. Paguridae.—The abdomen is asymmetrical, with some of the limbs lost. The antennal scale is well developed, and the flagella of the first antennae end in a filament.

Sub-Fam. 1. Eupagurinae.—The third maxillipedes are wide apart at the base, and the right chelipedes are much larger than the left. Parapagurus from deep-sea, Eupagurus from temperate, especially north temperate seas. Pylopagurus.

Sub-Fam. 2. Pagurinae.—The third maxillipedes are approximated at the base; the chelipedes are equal or subequal, or the left is much larger. Chiefly in the warm and tropical seas, but Clibanarius and Diogenes also in the Mediterranean.

Fam. 3. Cenobitidae.—The abdomen is as in Paguridae. The antennal scale is reduced, the flagella of the first antennae end bluntly. The members of this family are characteristic of tropical beaches, where they live on the land. Cenobita, with about six species, in the West Indies and Indo-Pacific, living in Mollusc shells; Birgus (Fig. ) on Indo-Pacific coral islands.

Fam. 4. Lithodidae.—The abdomen is bent under the thorax, and the body is crab-like and calcified. The rostrum is spiniform, and the sixth abdominal appendages are lost.

Sub-Fam. 1. Hapalogasterinae.—Abdomen not fully calcified, and without complicated plates. Hapalogaster and Dermaturus in the North Pacific littoral.

Sub-Fam. 2. Lithodinae.—Abdomen fully calcified, with a complicated arrangement of plates. Lithodes (Fig. ) practically universal distribution, littoral and deep sea. Acantholithus, deep littoral of Japan; Paralomis, west coast of America. This last genus should probably be placed in a separate family.

Sub-Order 3. Brachyura.

The abdomen is much reduced, especially in the male, and is carried completely flexed on to the ventral face of the thorax so as to be invisible from the dorsal surface. The pleopods in the male are only present on the two anterior segments, and are highly modified as copulatory organs; the pleopods in the female are four in number and are used simply for carrying the eggs; the pleopods of the sixth pair are always absent in both sexes. The first antennae and the stalked eyes can be retracted into special pits excavated in the carapace.

The larva hatches out as a Zoaea (Fig. , A) very similar to that of the Anomura; it is furnished with an anterior and posterior spine on the carapace. It is characteristic of the Brachyuran Zoaea that the third maxillipede is fashioned from the beginning in its definitive expanded form, and is never a biramous swimming organ as in the Anomura. The only exception to this rule is found in the Dromiacea, the most primitive of the Brachyura, to be soon considered, in which not only the third maxillipede, but also the first pair of pereiopods may be developed as biramous oars, a condition taking one back to the Mysis stage of the Macrura. The Metazoaea (Fig. , B) has the rudiments of the thoracic limbs developed and crowded together at the back of the carapace; they are all laid down in their definitive forms, and the abdomen has the pleopods precociously developed. These Zoaeal stages are of course pelagic, but the Metazoaea next passes into the Megalopa stage (Fig. ), in which the little crab forsakes its pelagic life and assumes the ground-habits of the adult; the Megalopa, which corresponds exactly to the Glaucothoe of the Pagurids, resembles a small Galathea or Porcellana, the abdomen being still large and unflexed and furnished with normal pleopods. From this stage the adult structure is soon achieved, though, owing to the continued growth of the Crustacea even after maturity is reached, there is often a slight progressive change in structure, especially in the male, at each successive moult of the individual. The Megalopa of Corystes cassivelaunus is peculiar in the immense production of the second antennae, which act as a respiratory tube (Fig. ).

The Brachyura must be considered under the following subdivisions:—

Tribe 1. Dromiacea.

All authorities are agreed that these are the most primitive of the Brachyura. In them the abdomen is much less reduced in both sexes than in other Brachyura; there is a common orbitoantennary fossa, into which eyes and antennae are withdrawn, instead of a separate one on each side for each organ; the carapace is often much elongated as in the Macrura and Anomura, and a number of other anatomical characters might be mentioned which characterise the Dromiacea as intermediate between the true Brachyura and the lower forms. There are, however, two views as to the relationship of the Dromiacea; Claus held that they proceeded from a Galatheid stock, and hence that the development of the Brachyura ran through an Anomurous strain; but Huxley, and latterly Bouvier, adopt the view that the Dromiacea are descended, not from the Galatheidae, but direct from the Macrura, and especially from the Nephropsidea. Special resemblances are found between the Jurassic Nephropsidae and certain present day Dromiacea, e.g. Homolodromia paradoxa, the detailed form of the carapace in the two cases being very similar. It is, however, a little strange that in the Dromiacea we meet with the same reduction and dorsal position of the last, or last two pairs of thoracic limbs which we saw to be such a characteristic feature of the Anomura, especially of the Galatheidae. In the Dromiacea these limbs may be chelate, and they are used for attaching shells and other bodies temporarily to the back. Must we suppose that this resemblance to the Anomura is due to convergence, or that the Nephropsidae, which gave rise to perhaps both Galatheidae and Dromiacea, had this character, and that it has been subsequently lost in the Macruran stock? We have already mentioned that the Metazoaea of Dromia has not only a well-developed swimming third maxillipede, but also a biramous first pereiopod, a character which speaks strongly for Macruran affinities.

Fam. 1. Dromiidae.—The eyes and antennules are retractile into orbits. The last two pairs of thoracic limbs are small, and held dorsally. The sixth pair of pleopods are rudimentary or absent. Homolodromia from West Indies, deep-sea. Dromia, widely dispersed. D. vulgaris (Fig. ) occurs on the English coasts.

Fam. 2. Dynomenidae.—Similar to the preceding family, but only the last pair of thoracic limbs is small, and held dorsally. The sixth pair of pleopods are reduced, but always present. Dynomene in the Indo-Pacific.

Fam. 3. Homolidae.—The eyes and antennules are not retractile into orbits. Only the last pair of thoracic limbs are reduced, the sixth pair of pleopods altogether absent. Homola and Latreillia, widely distributed, occur in the Mediterranean. Latreillopsis from the Pacific. L. petterdi, a magnificent species, with the carapace nearly a foot long, and with very long legs like a Spider-crab, has been dredged from 800 fathoms east of Sydney, New South Wales.

Tribe 2. Oxystomata.

This group comprises Crabs whose carapace is more or less circular, while the mouth, instead of being square as in the remaining Brachyura, is triangular with the apex pointing forward, and the third maxillipedes are not expanded into the flattened, lid-like structures found in other Crabs. There is the same tendency in some of the genera for the posterior thoracic limbs to be reduced and carried dorsally, as in the Galatheidae and Dromiacea. The well-known Dorippe from the Mediterranean has this feature, and frequently carries an empty shell upon its back, and Cymonomus presents the same peculiarity.

Cymonomus granulatus (Fig. ) is an abyssal form that has been dredged from the Mediterranean and North Atlantic, in which the eye-stalks are curiously tuberculated, and the ommatidia of the eye are entirely unpigmented and degenerate, though a few corneal facets are still recognisable. This species is replaced by C. quadratus in the Caribbean Sea and by C. normani on the East African coast, in which the alteration of the eye-stalks into thorny, beak-like projections becomes progressively marked, and all traces even of the corneal facets disappear. This remarkable genus was mentioned in the excursus on Crustacean eyes on p. 149.

The Oxystomata, like the Cyclometopa, to be considered later, live in sandy and gravelly regions, and burrow to a greater or less extent, and we find in both groups admirable adaptations for securing a pure stream of water, uncontaminated by particles of sand, for flushing the gills. Perhaps the most remarkable of these adaptations is afforded by Calappa. This animal has the chelipedes wonderfully modified in structure, and when it is reposing in the sand it holds them apposed to the front of the carapace, as shown in Fig. , so that the spines upon their edges, together with the hairy margin of the carapace, form a most efficient filter for straining off sand and grit from the stream of water which is sucked down between the closely-fitting chelipedes and carapace, to enter the branchial chambers at their sides. The exhaled current of water passes out anteriorly through a tube formed by a prolongation of the endopodites of the first maxillipedes. The exhalant aperture is shown in Fig. by the two black cavities below the snout in the middle line.

A similar method is pursued by the related Matuta banksii (Fig. ), a swimming and fossorial Crab found in the Indo-Pacific. In this Crab the chelipedes also fit against the carapace to form a strainer, and their function is assisted by the enlargement of the posterior spine, which acts as a kind of elbow-rest to keep the chelipedes properly in position. The inhalant openings are situated just in front of the chelipedes. It is a most remarkable fact that among the Cyclometopa, Lupa hastata (Fig. ) has an exactly similar arrangement. Apparently we have here another instance of convergence, similar to that of Corystes and Albunea, but the case is complicated by the fact that some of the Oxystomata, and among them Matuta, show a certain amount of relationship to the Cyclometopous Portunids, so that it is just conceivable that the resemblances in the respiratory arrangement are due to a common descent and not to convergence.

In the Leucosiidae, of which the Mediterranean Ilia nucleus (Fig. ) is an example, the inhalant aperture is situated between the orbits, and leads into gutters excavated in the “pterygostomial plates” flanking the mouth, which are furnished with filtering hairs and are converted into closed canals by expansions of the exopodites of the third maxillipedes. Thus these Crabs possess a filtering apparatus independent of the chelipedes and of the margin of the carapace.

Fam. 1. Calappidae.—Cephalothorax rounded and crab-like. The abdomen is hidden under the thorax, the antennae are small, and the legs normal in position. The afferent openings to the gill-chambers lie in front of the chelipedes. Male openings on coxae of last pair of legs. Calappa (Fig. ) circumtropical, and extending into the warmer temperate seas. Matuta (Fig. ) from the Indo-Pacific.

Fam. 2. Leucosiidae.—Similar to the above, but the afferent openings to the gill-chambers lie at the bases of the third maxillipedes. Male openings on the sternum. This family contains a great number of forms, with headquarters in the tropical littoral, but extending into the temperate seas. Ilia in the European seas. I. nucleus (Fig. ) common in the Mediterranean. Ebalia in the Atlantic, North Sea, and Indo-Pacific. Leucosia in Indo-Pacific.

Fam. 3. Dorippidae.—Cephalothorax short and square. The abdomen is not hidden under the thorax; the antennae are large, and the last two pairs of legs are held dorsally, and have terminal hooked claws. Dorippe, littoral in Mediterranean and Indo-Pacific. Cymonomus (Fig. ) from deep-sea of Atlantic and Mediterranean.

Fam. 4. Raninidae.—Similar to Dorippidae, but the cephalothorax is elongated, and the legs usually have the last two joints very broad. Several genera, chiefly in the deeper littoral zone. Ranina dentata in the Indo-Pacific.

Tribe 3. Cyclometopa.

In these Crabs the carapace is circular rather than square; its frontal and lateral margins are produced into spines and there is no pointed rostrum. The mouth is square, and the third maxillipedes are greatly flattened and form a lid-like expansion over the other oral appendages. This group includes the common Shore-crab of our coasts (Carcinus maenas), the swimming Crabs with expanded pereiopods (Portunus, Lupa, etc.), the Edible Crab (Cancer pagurus), and many others.

Corystes cassivelaunus is a Crab of doubtful affinities. It is sometimes placed among the Oxyrhyncha, but, as Gurney has pointed out, the Megalopa shows Portunid characters, and the resemblance to the Oxystomata in the front of the carapace and in the mouth may be secondary. The respiratory arrangement of this Crab has already been mentioned in comparing its structure with that of the Mole-crab Albunea. The form of the antennal tube can be gathered from the figure of the Megalopa stage (Fig. , p. 183). It should be noted that when the Crab is buried in the sand with only the tip of the antennal tube projecting, the water is sucked down and enters the branchial cavities anteriorly, the antennal tube being continued by a tube formed from the third maxillipedes and the forehead; the water is exhaled at the sides of the branchial cavities beneath the branchiostegites. Thus in Corystes the normal direction of the current is reversed, but when the Crab is not buried, and is moving over the surface, it breathes in the usual manner, taking in the water at the sides of the branchiostegites and exhaling it anteriorly by the tube. The related Atelecyclus, found like Corystes very commonly at Plymouth, uses two methods of breathing: when it is in the surface-layers of sand it makes use of its antennal tube, which is, however, much shorter than in Corystes; but when it burrows deeper, where the antennal tube is no use, it folds its chelipedes and also its other legs, which are densely covered with bristles, so as to form a reservoir of pure water underneath it free from sand, which it passes through the gill-chambers in the usual manner (see Garstang, loc. cit. p. 186).

The respiratory adaptations in Lupa hastata and their convergence towards those of the Oxystomatous Matuta have been already touched upon (pp. 186, 187).

In this connexion must be mentioned the interesting experiments of W. F. R. Weldon upon the respiratory functions of Carcinus maenas at Plymouth, since these were the first noteworthy observations directed towards the exact measurement of the action of natural selection upon any animal, a field of observation in which Weldon will always be looked upon as a pioneer. An extended series of measurements by Weldon and Thompson on male specimens of Carcinus maenas of various sizes between the years 1893 and 1898 showed a steady decrease in the ratio of carapace breadth to length; the Crabs appeared to be becoming steadily narrower across the frontal margin, and the same thing, though not to the same extent, was happening in female Crabs. Weldon supposed that this change might be correlated with the silting up of Plymouth Sound and the consequent fouling of the water. To test this hypothesis he kept a very large number of male Crabs in water to which fine porcelain clay was added and kept in continual motion. In the course of the experiments the survivors and the dead were measured, and it was found that the mean carapace breadth of the survivors was less than that of those that succumbed. The experiment was repeated with the fine sand that is deposited and left at low water upon the stones on Plymouth beach, and the same result was observed. It was also noticed that the individuals which died had their gills clogged with the sand, while those that survived had not. As a further confirmation, a great many young male Crabs were isolated and kept in pure filtered water, and they were measured before and after moulting; these measurements, when compared with measurements of the frontal breadth in Crabs of the same size taken at random upon the beach, were found to show a greater breadth than the wild Crabs, thus indicating that a selection of narrow Crabs was taking place in Nature which did not take place when the Crabs were protected from the effects of fine sand in the water.

The whole chain of evidence goes to show that the carapace breadth in Carcinus maenas in Plymouth Sound is being influenced by the rapid change of conditions occurring in the locality. Various objections have been urged against this conclusion, but, though they merit further investigation, they do not appear very weighty.

The fresh-water Crab, Thelphusa fluviatilis, common in the South of Europe and on the North coast of Africa, belongs to the Cyclometopa, and is interesting from its direct mode of development without metamorphosis.

Fam. 1. Corystidae.—The orbits are formed, but, unlike all the other families of the Cyclometopa, are incomplete. The body is elongate and oval, and the rostrum and front edge of the mouth rather as in the Oxyrhyncha, in which Tribe they are sometimes included. Corystes, with a few species in European seas. C. cassivelaunus at Plymouth.

Fam. 2. Atelecyclidae.—Perhaps related to the foregoing. The carapace is sub-circular, and the rostrum short and toothed. Atelecyclus, European seas.

Fam. 3. Cancridae.—The carapace is broadly oval or hexagonal, and the flagella of the second antennae are short and not hairy as in the foregoing. The first antennae fold lengthwise. Carcinus maenas on English and North European coasts. This crab has become naturalised in some unexplained manner in Port Phillip, Melbourne. Cancer in North Atlantic, North Pacific, and along the west coast of America into the Antarctic regions. C. pagurus is the British Edible Crab.

Fam. 4. Portunidae.—The legs are flattened and adapted for swimming. The first antennae fold back transversely. Portunus, Atlantic and Mediterranean. Neptunus, Indo-Pacific. Callinectes, C. sapidus, the edible blue Crab of the Atlantic coasts of America. Lupa (Fig. ).

Fam. 5. Xanthidae.—The first antennae fold transversely, but the legs are not adapted for swimming; the body is usually transversely oval. This family is especially characteristic of the tropical littoral, where it is very widely represented. Xantho, Actaea, Chlorodius, Pilumnus, Eriphia, with E. spinifrons, common in the Mediterranean.

Fam. 6. Thelphusidae (Potamonidae).—Fresh-water crabs, with the branchial region very much swollen. Thelphusa (or Potamon) has nearly a hundred species distributed from North Australia, through Asia, Japan, the Mediterranean region, and throughout Africa. Potamocarcinus in tropical America.

Tribe 4. Oxyrhyncha.

This section includes the Spider-crabs and related genera, in which the carapace is triangular, with the apex in front formed by a sharply-pointed rostrum. There are two chief series, the one comprising the Spider-crabs, with much elongated walking legs, e.g. the huge Maia squinado of European seas, the yet more enormous Macrocheira kämpferi from Japan, supposed to be the largest Crustacean in existence, and sometimes spanning from outstretched chela to chela as much as eleven feet, and the smaller forms, such as Inachus, Hyas, and Stenorhynchus, which are so common in moderate depths off the English coasts. The other series is represented by genera like Lambrus (Fig. ), in which the legs are not much elongated, but the chelipedes are enormous.

The Spider-crabs do not burrow, and their respiratory mechanism is simple; but since they are forms that clamber about among weeds, etc., upon the sea-bottom, they often show remarkable protective resemblances to their surroundings, which are not found in the burrowing Cyclometopa. Alcock gives a good account and figure of Parthenope investigatoris, one of the short-legged Oxyrhyncha, the whole of whose dorsal surface is wonderfully sculptured to resemble a piece of the old corroded coral among which it lives.

But besides this, the long-legged forms, such as Inachus, Hyas, etc., have the habit of planting out Zoophytes, Sponges, and Algae upon their spiny carapaces, so that they literally become part and parcel of the organic surroundings among which they live. It may, perhaps, be wondered what are the enemies which these armoured Crustacea fear. Predaceous fish, such as the Cod, devour large quantities of Crabs, which are often found in their stomachs; and Octopuses of all sorts live specially upon Crabs, which they first of all paralyse by injecting them with the secretion of poison-glands situated in their mouth. The poison has been recently found by Dr. Martin Henze at Naples to be an alkaloid, minute quantities of which, when injected into a Crab, completely paralyse it. When the Crab is rendered helpless the Octopus cuts out a hole in the carapace with its beak, and sucks all the internal organs, and then leaves the empty shell.

Many of the Oxyrhyncha are found in the abysses; among them are Encephaloides armstrongi (Fig. ), dredged by Alcock from below the 100–fathom line in the Indian Ocean, which has the gill-chambers (G) greatly swollen and enlarged to make up for the scarcity of oxygen in these deep regions.

Fam. 1. Maiidae.—The chelipedes are not much larger than the other legs, but are very mobile. Orbits incomplete. A very large family, including all the true Spider-crabs, very common in the Atlantic and Mediterranean littoral. Inachus, Pisa, Hyas, Stenorhynchus, Maia, Encephaloides (Fig. ).

Fam. 2. Parthenopidae.—The chelipedes are much larger than the other legs. Orbits complete. Lambrus (Fig. ), Parthenope.

Fam. 3. Hymenosomatidae. The carapace is thin and flat; the chelipedes are neither very long nor especially mobile. There are no orbits, and the male openings are on the sternum. Characteristic of the Antarctic seas. Hymenosoma, Trigonoplax.

Tribe 5. Catometopa.

These Crabs resemble the Cyclometopa in general appearance, but the carapace is very square in outline, and its margins are never so well provided with spines as in the Cyclometopa. The position of the male genital openings is peculiar, since they lie upon the sternum, and are connected with the copulatory appendages upon the abdomen by means of furrows excavated in the sternum. The Catometopa are either littoral or shallow water forms, or else they live entirely on land. The Grapsidae are marine Crabs, Pachygrapsus marmoratus (Fig. ) at Naples being exceedingly common on rocks at high-water mark, over which it scuttles at a great rate; in the Mediterranean it takes the place of our common Garcinus maenas, which is not found there.

Among the land genera are Ocypoda, Gelasimus, and Gecarcinus of tropical lagoons and coastal swamps. Ocypoda often occurs in vast crowds in these regions, and digs burrows in the sand.

Gelasimus (Fig. ) is remarkable for the enormous size of one of the chelipedes, generally the right, in the male, which may actually exceed in size the rest of the body. It is not known what purpose this organ serves in the various species. In Gelasimus it is supposed that the male stops up the mouth of the burrow with it when he and the female are safely inside. It is also used as a weapon in sexual combats with other males; but Alcock, from observations made in the Indian Ocean, believes that the males use it for exciting the admiration of the females in courtship, as the huge chela is bright red in colour, and the males brandish it about before the females as if displaying its florid beauty.

The species of Ocypoda are exclusively terrestrial, and cannot live for a day in water. The gills have entirely disappeared, and the branchial chambers are converted into air-breathing lungs with highly vascular walls, the entrances into which are situated as round holes between the bases of the third and fourth pairs of walking legs. As their name implies, they can run with astonishing rapidity, and they seem to be always on the alert, directing their eyes, which are placed on exceedingly long stalks, in all directions.

Some of the Grapsidae, e.g. Aratus pisonii, are partially adapted for life on land. Fritz Müller, in his Facts for Darwin, alludes to this creature as “a charming lively crab which ascends mangrove bushes and gnaws their leaves.” The carapace can be elevated and depressed posteriorly, apparently by means of a membranous sac, which can be inflated by the body-fluids. This Crab retains its gills and can breathe under water in the ordinary way.

A great many other Catometopa are land-crabs; but we may specially mention the genus Gecarcinus, related to the marine Grapsidae, which has representatives in the West Indies and West Africa. The Crabs of this genus may live in sheltered situations several miles from the sea, but in spring the whole adult population rushes down in immense troops to the shore, where breeding and spawning take place; and when this is completed they migrate back again to the land. The young pass through the normal larval stages in the sea and then migrate inland.

Fam. 1. Carcinoplacidae.—The carapace is rounded and broader than long, usually with toothed front margin. The orbits and eyes are normal, and not much enlarged. Geryon, in the deep littoral of the northern hemisphere. Euryplax, Panoplax, etc., in the American coastal waters. Typhlocarcinus, etc., in the Indo-Pacific.

Fam. 2. Gonoplacidae.—The carapace is square, with the antero-lateral corners produced into spines. The orbits are transversely widened, and the eye-stalks long. Gonoplax, widely distributed in the littoral zone. G. rhomboides in British and European seas.

Fam. 3. Pinnotheridae.—Carapace round, with indistinct frontal margin. Orbits and eyes very small, often rudimentary. The members of this family live symbiotically or parasitically in the shells of living Bivalve Molluscs, corals, and wormtubes in all seas except the Arctic. Pinnotheres pisum is fairly commonly met with off the English coasts in the mantle-cavity of Cardium norwegicum.

Fam. 4. Grapsidae.—Carapace square, the lateral margins either strictly parallel or slightly arched. The orbits and eyes are moderately large, but the eye-stalks are not much lengthened. Littoral, fresh-water, and land. Pachygrapsus marmoratus (Fig. ), the common shore-crab of the Mediterranean. Sesarma, with fresh-water and land representatives in the tropics of both hemispheres. Cyclograpsus, marine in the tropical littoral.

Fam. 5. Gecarcinidae.—Carapace square, but much swollen in the branchial region. Orbits and eyes moderately large. Typically land forms, which only occasionally visit the sea or fresh water. Cardisoma is a completely circumtropical genus, with species in tropical America, West and East Africa, and throughout the Indo-Pacific. Gecarcinus in West Indies and West Africa.

Fam. 6. Ocypodidae.—Carapace square or rounded, generally without teeth on the lateral margins. The orbits transversely lengthened, eye-stalks usually very long. The members of this family generally inhabit the mud-flats and sands of tropical coasts; in the southern hemisphere they extend far into the temperate regions. Macrophthalmus, with numerous species, in Indo-Pacific. Gelasimus (Fig. ), in the tropics of both hemispheres. Ocypoda, with similar distribution.

Chapter VII. Remarks on the Distribution of Marine and Fresh-Water Crustacea

A. Marine.

The great majority of the Crustacea are inhabitants of the sea. From a Zoogeographical point of view we divide the sea into three chief regions, each of which is characterised by a special kind of fauna—the littoral, the pelagic, and the abyssal regions.

The littoral region, which comprises all the shallow coastal waters down to about 100 fathoms, varies very greatly in its physical character according to the nature of the coast, its geological constitution, latitude, etc., but, on the whole, it is characterised by variability of temperature and salinity, by the presence of sunlight, and by the continuous motion of its waves. On the shores of the large oceans this region is also greatly affected by the tides. It is inhabited by a vast assemblage of Crustacea, all of which are dependent upon a solid substratum, either of rock or sand, or of vegetable or animal growth, upon which they may wander in search of food, or in which they may hide themselves. In consequence, the character of the Crustacea on any shore is largely determined by its geological nature.

Although a certain number of Entomostraca such as Copepoda (Harpacticidae and Cyclopidae), Ostracoda (Cypridae and Cytheridae), and a few Operculata are littoral in habit, it is the Malacostraca, from their larger size and variety of form, which give the character to coastal waters.

On rocky coasts, especially those affected by tides, a great many kinds of Shore-crab are found, which hide at low tide in the rock-pools and under stones. Carcinus maenas is characteristic of the rocky coasts of the North Sea, while it is replaced in warmer seas and all round the tropics by Crabs of the family Grapsidae, which are typical rock-livers, and exceedingly agile in clambering over tide-washed rocks. Porcellanidae are also very common under stones at low tide on rocky beaches. Such typical Shore-crabs as these are remarkably resistant to desiccation, and can live out of water for an astonishing time; nor do they require a change of water provided they have access to the air. The edible crab (Cancer pagurus) and the lobsters (Homarus and Palinurus) are dependent on rocks, but they rarely come close in-shore, preferring depths of a few fathoms.

Sandy coasts are preferred by Shrimps and Prawns, which haunt the shallow coastal waters in shoals; and in the sand are found all the Crabs whose respiratory mechanism is specially adapted for life in these regions, e.g. Hippidea or Mole-crabs, Corystes, Matuta, Calappa, etc.

Characteristic of sandy bottoms are also the Thalassinidea, such as Callianassa, which excavate galleries in the sand. On tropical sandy shores various species of Ocypoda and Gelasimus are conspicuous, which have deserted the sea, and live in burrows which they excavate on the shore. Gelasimus is especially abundant in the muddy sand of tropical mangrove swamps.

Besides the rocky and sandy coasts we must distinguish the muddy shores and bottoms which support a large amount of vegetable and animal growth. These, besides harbouring the greater number of Amphipods and Isopods, are also the natural home of the Dromiacea and Oxyrhyncha, or Spider-crabs, among which the habit is common of decking themselves out with pieces of weed or animal growth in order to harmonise better with their surroundings. Pagurids are also especially abundant in the deeper waters of these coasts.

Coral-reefs support a characteristic Crustacean fauna. In the growing coral at the reef-edge a number of small Cyclometopa are found, e.g. Chlorodius, Actaea, Xantho, which are finely sculptured and often coloured so as to harmonise with the coral. Alpheidae also, Shrimp-like Macrura with highly asymmetrical claws, which can emit a sharp cracking sound with the larger claw, are commonly found in pools on the reef. In the coralshingle formed by abrasion from the reef-edge at a few fathoms depth, Leucosiidae are found, in which, again, respiratory mechanisms for filtering sand from the gills are present.

Besides the geological nature of the coast, latitude has a very important bearing upon the distribution of littoral Crustacea. Indeed, the present distribution of littoral Crustacea appears to be far more determined by the temperature of the coastal waters than by the presence of any land-barriers, however formidable. We may distinguish an Arctic, Antarctic, and Circumtropical zone.

The Arctic zone includes the true Arctic seas, and stretches right down through boreal regions towards the sub-tropical seas. Almost all the truly Arctic forms penetrate fairly far south, the Arctic seas being characterised more by the absence of temperate forms than by the presence of forms peculiar to itself. At the same time it must be noted that the individuals from the coldest regions often grow to an enormous size, a characteristic which is physiologically unexplained.

A great many of the Crustacea characteristic of this region are circumpolar, i.e. they are not restricted in range to either the Atlantic or Pacific. This is especially true of the extremely northern types, e.g. Crangonidae and Hippolytidae, but it is also true of a number of Crustacea which do not now occur as far north as Greenland or Bering Strait, so that there is no longer any free communication for them between Pacific and Atlantic. This gives rise to a discontinuous distribution in the two oceans, exemplified in the common Shrimp, Crangon vulgaris, which is found on the temperate European coasts and on the Pacific coasts of Japan and Eastern America. The same is true of Eupagurus pubescens and E. bernhardus.

At the same time the boreal Atlantic and Pacific have their peculiar forms. Thus the European and American Lobsters are confined to the Atlantic, while the North Pacific possesses a very rich array of Lithodinae, which cannot be paralleled in the Atlantic.

We may explain the community of many littoral forms to both the North Atlantic and Pacific coasts by the continuous coast-line uniting them, which in former times possibly did not lie so far north, or else was not subjected to so rigorous a climate as now.

In the Antarctic zone we are presented with very different relations, since the great continents are drawn out to points towards the south, and are isolated by vast tracts of intervening deep sea. Nevertheless, certain littoral forms are circumpolar, e.g. the Palinurid Iasus and the Crabs Cyclograpsus and Hymenosoma. The genus Dromidia is common to Australia and South Africa, though it is apparently absent from South America.

The Isopod genus Serolis is confined to Antarctic seas. The majority are littoral species, and they are distributed round the coasts of Patagonia, Australia, and Kerguelen in a manner that certainly suggests a closer connection between these shores in the past. These facts are, on the whole, evidence in favour of the former existence of an Antarctic continent stretching farther north and connecting Australia, Africa, and S. America—a supposition that has been put forward to account for the distribution of the Penguins, Struthious birds, Oligochaets, Crayfishes, etc., in these regions (see pp. –217).

In considering the Arctic and Antarctic faunas the supposed phenomenon of bipolarity must be mentioned, i.e. the occurrence of particular species in Arctic and Antarctic seas, but not in the intermediate regions. This discontinuous type of distribution was upheld for a variety of marine animals by Pfeffer, Murray, and others, but it has been very adversely criticised by Ortmann. As far as the Arctic and Antarctic Decapod fauna in general are concerned, the north polar forms are quite distinct from the south polar. Typical of the former are Hippolyte, Sclerocrangon, Hyas, Homarus, etc.; of the latter, Hymenosoma, Dromidia, Iasus. It appears, however, that in certain special cases, bipolarity of distribution may be produced owing to the operation of peculiar causes. Two such cases seem to be fairly well established. Crangon antarcticus occurs at the two poles, and apparently not in the intermediate regions; but, as Ortmann points out, it is represented right down the West American coast by a very closely related form, C. franciscorum. The waters on the tropical western coasts both of Africa and America are exceedingly cool, and it appears that in this way the Crangon may have migrated across the tropical belt, leaving a slightly modified race to represent it in this intermediate region. The other case of bipolarity is afforded by the “Schizopod,” Boreomysis scyphops, which occurs at both poles, but is not known from the tropics. This is a pelagic species, and we know that the Mysidae often descend to considerable depths. We also know that the Mysidae are dependent on cold water, only occurring in boreal or temperate waters. We may safely suppose, therefore, that the migration of this species has taken place by their forsaking the surface-waters as the tropics were approached, and passing down into the depths where the temperature is constantly low even in the tropics.

The dependence of Crustacea upon the temperature of the water is also illustrated by the distribution of the Lithodinae. The headquarters of this family are in the boreal Pacific, with a few scattered representatives in the boreal Atlantic. The cool currents on the western coasts of America, however, have permitted certain forms to migrate as far south as Patagonia, where they still have a littoral habit. In the tropical Indo-Pacific, where a few species occur, they are only found in deep waters. Thus at these various latitudes, by following cool currents or migrating into deep water, they are always subjected to similar conditions of temperature. The same kind of thing is observed in Arctic seas, where deep-sea forms are apt to take on secondarily a littoral habit owing to the temperature of the depths and of the shore being the same.

Despite the impassable barriers of land which now sever the tropical oceans, we can yet speak of a circumtropical zone possessing many species common to its most widely separated parts. Such circumtropical species, occurring on both the Atlantic and Pacific coasts of tropical America, on the West African coast, and in the Indo-Pacific, are various Grapsidae, Calappa granulata and its allies, and certain Albunea. The most striking instance of all is that of the Land-crabs. Of Ocypoda, the greater number of species occur in the Indo-Pacific, but representatives are also found on the tropical Eastern and Western American coasts and on the West African coast, and the same is true of Gelasimus. The genus Cardisoma, belonging to a different group of Land-crabs, is also typically circumtropical.

For this community of the circumtropical species we may certainly advance in explanation the comparatively recent formation of the Isthmus of Panama. Besides the resemblance of the Crustacea on the east and west coasts of the isthmus, we have an actual identity of species in several cases, e.g. Pachycheles panamensis and Hippa emerita, and the same thing has been observed for the marine fish.

Another connexion, at any rate during early tertiary times, which probably existed between now isolated tropical coasts, was across the Atlantic from the West Indies to the Mediterranean and West African coasts. Numerous facts speak for this connexion. Species of Palinurus and Dromia occur in the West Indies and the Mediterranean, which only differ from one another in detail, and a connexion between these two regions has been urged from the minute resemblances of the late Cretaceous Corals of the West Indies with those of the Gosau beds of S. Europe, and also of the Miocene land-molluscs of S. Europe with those at the present time found in the West Indies.

To account, then, for the present distribution of littoral Crustacea we must imagine that great changes have taken place during comparatively recent times in the coast-lines of the ocean, but the guiding principle in both the past and present has been temperature, and this factor enables us, despite the immense changes in the configuration of the globe that must have taken place, to divide the coasts latitudinally into Arctic, Antarctic, and Circumtropical zones.

Pelagic Crustacea belong chiefly to the Copepoda (Calanidae, Centropagidae, Candacidae, Pontellidae, Corycaeidae), a few Ostracoda (Halocypridae and Cypridinae), and among Malacostraca a few Amphipoda (Hyperina), some “Schizopoda,” and among Decapoda only the Sergestidae, if we except the few special forms which live on the floating weeds of the Sargasso Sea, e.g. the Prawns Virbius acuminatus and Latreutes ensiferus, and the Brachyura Neptunus sayi and Planes minutus. Besides these Crustacea which are pelagic as adults, there is an enormous host of larval forms, both among Entomostraca and Malacostraca, which are taken in the surface-plankton.

In dealing with the Copepoda we have already mentioned the vast pelagic shoals of these organisms which occur at particular times of the year, and have an important influence on fishing industries. Anomalocera pattersoni (Fig. , p. 60) is a good instance of this. It is a large Heterarthrandrian, about 3 mm. long, with the body of a fine bluish green colour; it has a remarkable power of springing out of the water, so that a shoal has the appearance of fine rain upon the surface of the sea. It occurs in the open Atlantic and Mediterranean, but comes into the coasts during violent storms; the Norwegian fishermen hail its presence in the fjords as the sign of the approach of the summer herring.

It was Haeckel who first clearly distinguished between “neritic” plankton, the species of which have their centres of distribution in shallow coastal waters and die out gradually as the open ocean is approached, and “oceanic” plankton which is habitually found in the open sea, and though it may invade the coasts is not dependent on the sea-bottom in any way. It appears that although these two kinds of plankton may get mixed up by currents and storms, they are always recruited by new generations from the neritic or oceanic stations proper to each kind.

Common oceanic species, found chiefly in the open Atlantic and in the North Sea, are Anomalocera pattersoni, Calanus finmarchicus, Centropages typicus, Metridia lucens, Oithona plumifera, etc. Common neritic species in the Channel and other coastal waters are Centropages hamatus, Euterpe acutifrons, Oithona nana, Temora longicornis, etc. It was found by Gough that although the true oceanic species invade the Channel from the open Atlantic to the west, they become rarer and rarer as they advance up the Channel. Thus the plankton midway between the Lizard and Ushant at all times of year is about 70 per cent. oceanic, while at the line drawn from Portland to the Cap de la Hague it is about 35 per cent. Seasonal changes in the salinity of the Channel water, chiefly due to the influx of oceanic water from the Atlantic, as observed by Matthews, do not clearly influence the distribution of oceanic and neritic forms. The influx of highly saline water from the Atlantic was most marked during the winter months up to February. From February to May the highly saline water receded, and during the summer months at the line drawn between Portland and the Cap de la Hague the salinity was rather low. This was increased in November by a patch of oceanic water being cut off from the main mass and passing up Channel, and it is noteworthy that during this month the highest percentage of oceanic forms was taken in the plankton of this region.

Calanus finmarchicus affords a clear instance of the way in which the plankton may be carried about for great distances by means of currents. This species has its home in the subarctic seas, but is carried down in the spring by the East Icelandic Polar stream to its spawning-place south of Iceland; the enormous shoals produced here are carried back, continually multiplying, along the coasts of Norway during the summer and autumn.

Besides these great migrations, the plankton organisms perform daily movements, the majority of the Crustacea avoiding the surface during the day, and often going down to as much as seventy fathoms or more, and only coming up to the surface at night. Others, however, e.g. Calanus finmarchicus, behave in the converse manner, preferring the sunlit surface to swim in.

Owing to their dispersal by means of oceanic currents the pelagic Crustacea do not offer any very striking features in regard to their distribution, and the possibility of always finding congenial temperatures by passing into the upper or under strata of water enables them to live in almost all seas. The tropical species of Sergestidae are mostly circumtropical, i.e. unhindered by the present barriers of land.

The Abyssal regions of the sea contain many of the most interesting Crustacea. Families entirely confined to the abyss are the Eryonidae, Pylochelidae, and certain Caridean Prawns (Psalidopodidae, etc.), but there are a great number of normally littoral genera which have representatives in deep water. If we draw the limit between the littoral and abyssal regions at about 200 metres, we can characterise the latter as absolutely dark except for the presence of phosphorescent organisms, with the temperature at a little above zero, and with a comparative lack of dissolved oxygen in the water. These conditions bring about remarkable modifications in the structure and life-histories of the inhabitants of the deep sea; we have already touched on the modifications of the visual organs and on the presence of phosphorescence in many of the animals; other points to be noticed are the usually uniform yellowish or bright red coloration, the frequent delicacy of the tissues without much calcification, variations in the structure of the breathing organs, e.g. in Bathynomus giganteus and Encephaloides armstrongi, and the loss of the larval development. Owing to the similarity of conditions in the deep sea all over the globe most of its inhabitants are universally distributed. It is also a striking fact that species are found in the deep sea of the tropics whose nearest allies occur, not in the littoral seas of the tropics, but in those of the temperate region. This fact has already been alluded to in dealing with the distribution of the Lithodinae. Alcock remarks that between 50–500 fathoms in the Indian Ocean are found Crabs such as Maia, Latreillia, and Homola, regarded as characteristic of the north temperate seas; the lobster Nephrops andamanica, taken at 150–400 fathoms, is closely allied to the Norwegian N. norwegica; and nine species of “Schizopoda,” which are certainly temperate forms, occur in the Indian Ocean at depths of 500–1750 fathoms.

B. Fresh-Water.

If we except the Crayfishes and River-crabs, the Crustacean fauna of running water is exceedingly poor, but in all standing fresh-water, from the smallest pond to the large lakes and inland seas, Crustacea, especially Entomostraca, are abundant and characteristic, and form an important item in the food of fresh-water fishes. In small ponds a vast assemblage of Cladocera is met with; these animals multiply with great rapidity by parthenogenesis, especially during spring and summer, but on the advent of untoward conditions sexual individuals are produced, which lay fertilised winter-eggs which lie dormant until favourable conditions again arise. As Weismann first pointed out, the frequency with which sexual individuals are produced in the various species is closely correlated with the liability of the water in which they live to dry up; so that the Cladocera which inhabit small ponds usually have at least two “epidemics” of sexual individuals, one during early summer and the other before the onset of winter.

Besides Cladocera, the Phyllopoda (e.g. Apus, Artemia, etc.) inhabit small pools; and also a great number of Cyclopidae. Of the other fresh-water families of Copepoda, viz. Centropagidae and Harpacticidae, inhabitants of small pieces of water are Diaptomus castor, as opposed to the other species of Diaptomus which are pelagic, and a number of Harpacticidae (Canthocamptus), the members of this family living in the weed or mud of either small ponds or else on the shores of the larger lakes. The greater number of Ostracoda are found in similar situations.

A district like the Broads of Norfolk, which consists partly of slowly-moving streams and partly of extensive stretches of shallow water, supports a Crustacean fauna intermediate in character between that found in small ponds and the truly pelagic fauna characteristic of deep lakes. A very complete list of the Crustacea of the Norfolk Broads, with an interesting commentary on their distribution, is given by Mr. Robert Gurney. We miss here the pelagic Cladocera, such as Leptodora, Bythotrephes, Holopedium, etc., which form so characteristic a feature of large lakes; at the same time, besides a rich development of the Cladocera, Cyclopidae, and Harpacticidae, which haunt the weeds and mud of shallow waters, we find such species as Polyphemus pediculus and Bosmina longirostris among Cladocera, which are otherwise confined to large bodies of water, and a few pelagic Diaptomus, e.g. D. gracilis. The fauna is also complicated in this district by the proximity to the sea and the frequently high salinity of the water, which allows a number of typically marine Copepods to pass up the estuaries and intermingle with typically fresh-water species; such are Eurytemora affinis among the Centropagidae, and several species of Harpacticidae (see p. ).

The large lakes of the world, such as the continental lakes of Europe and America, or of our own Lake District, reproduce on a small scale the varied conditions which appertain to the ocean—as in the ocean, we can recognise in these lakes a littoral, a pelagic, and an abyssal region. Our knowledge of the physiography of lakes is largely due to the classical work of Forel, and the following account of the physical conditions in the various regions is condensed from his book.

The littoral region is sharply marked off from the others by the relative instability of its physical conditions, owing to the agitation of its waters, the affluence of streams and drainage, and the constant changes of temperature. The water in this region generally contains a good deal of solid matter in suspension, while the shelving banks of the lake support a wealth of vegetable growth, both of Algae and of Phanerogams, down to about 20–25 metres. At this depth the daylight does not penetrate sufficiently to admit of the growth of green plants, so that this region marks the limit, both physical and biological, between the littoral and the abyssal zones. In this littoral region there flourish a great quantity of Entomostraca, most of which are also found in small ponds where similar conditions of life prevail—the pelagic species only penetrating rarely, and by accident, into its waters. At the beginning of July Mr. H. O. S. Gibson and myself found that the weedy littoral region of Grasmere contained almost entirely large quantities of the Cladoceran Eurycercus lamellatus, and a number of Cyclops fuscus and C. strenuus. In the littoral zone of large lakes, Amphipods, Isopods, and fresh-water shrimps may also be met with, but this applies more to the lakes of the Tropics and of the Southern Hemisphere.

The pelagic region is distinguished from the littoral by the greater purity and transparency of its waters, and by the relative stability of the temperature, the annual range, even at the surface, in Geneva being from 4°–20° C., while at 100 metres the water has a uniform temperature of 4° or 5° C. The upper strata are, of course, brightly illuminated, but at 20 metres there is hardly sufficient light for green plants to grow, and at 100 metres it is completely dark. The inhabitants of this region, known collectively as plankton, spend their whole life swimming freely in the water, sometimes at the surface and sometimes in the deeper strata. They consist chiefly of Diatoms, Protozoa, Rotifera, and Crustacea. The pelagic Crustacea, especially the Cladocera, are often the most curiously and delicately built creatures. Leptodora hyalina, which is quite transparent, is the largest of them, attaining to three-quarters of an inch in length, though Bythotrephes longimanus is nearly as large if we include the immense spine which terminates the body. Holopedium gibberum, which is the commonest of all in Grasmere lake, but not so frequently met with in the other English lakes, is peculiar in that its body is enveloped in a spherical mass of transparent jelly, sometimes a quarter of an inch in diameter, so that the contents of a tow-net jar full of Holopedium have something of the consistency of boiled sago. The enormous quantities in which these animals often occur during summer is very astonishing; but to be truly appreciated tow-nettings should be taken at the surface of the lake either during night-time when there is not much moonlight, or else on a dark still day when there is a quiet drizzle falling on the surface of the water. In bright sunshine the plankton passes below the surface into the lower strata, and can be usually taken by sinking the tow-net some 10–20 feet, or to even greater depths in the water. The exact reason of these periodic migrations out of the light, and their dependence on other physical conditions, such as temperature and the agitation of the water, is not clearly understood. It appears, however, that when the water is rough, plankton always passes into the deeper regions. Besides the species mentioned, the minute Bosminidae, whose trunked heads are suggestive under the microscope of elephants, and Polyphemus pediculus are among the commonest pelagic Cladocera, though neither Polyphemus nor Bythotrephes ever form shoals. The above-mentioned genera are characteristic of the larger lakes in the Northern Hemisphere. Our knowledge of the Crustacean plankton of tropical lakes and of those of the Southern Hemisphere is limited (but see p. ).

A very important constituent of lake-plankton is furnished by the Copepoda, especially of the genus Diaptomus. With the exception of Holopedium, by far the commonest Crustacean in Grasmere during July was found by Mr. Gibson and myself to be D. caeruleus.

At the same time a number of Cyclopidae, e.g. Cyclops strenuus, may occur in the pelagic region in considerable quantities, though they were never found by us in such numbers as Diaptomus.

The life-cycle of the pelagic Entomostraca has been studied in both the Cladocera and the Copepoda. In some of the Cladocera Weismann at first supposed that males had altogether disappeared, and that reproduction was entirely parthenogenetic. It appears, however, that all the pelagic species have at least one sexual period, namely, in the autumn, when resting eggs are produced which lie dormant during the winter. The pelagic Copepods may either produce resting eggs for the winter (Diaptomus), or else the winter is passed through in the Nauplius stage, the larvae hibernating in the mud until the spring (Cyclopidae).

We have so far only dealt with fresh-water Entomostraca. There are, in addition, a number of Malacostraca which inhabit fresh water, and some of these are found in the abyssal region of the great lakes, which must now be considered.

The physical conditions of the abyssal region are still more stable than those of the pelagic region, since the water is never disturbed, the bottom is always composed of a fine mud, the temperature is constant at 4°–5° C., and there is a total absence of light. It was hardly expected that animals would inhabit this region, until Forel discovered Asellus aquaticus in a depth of forty metres in the Lake of Geneva, and subsequently showed that quite a number of animals, including a Hydra, several worms, Molluscs, Crustacea, and larval Insects, may be found in these or even much greater depths.

The Crustacea of the abyssal region are two in number, and these have been found in a number of European lakes; Niphargus puteanus, a blind Amphipod closely allied to Gammarus; and Asellus forelii, allied to A. aquaticus and A. cavaticus, which may be either quite blind or else retain the rudiments of eyes.

These two Crustacea, under a practically identical form, are also found in the subterranean waters of Europe, and Forel considers that they have arrived in the abysses of the lakes from the subterranean channels, and are not derivatives of the littoral fauna.

Having completed our short review of lacustrine Crustacea, we may deal with the subterranean and cave Crustacea, which, as far as light and temperature are concerned, are subjected to very similar conditions to those dwelling at the bottom of deep lakes. The inhabitants of the subterranean waters have been chiefly brought to light in Artesian wells, etc., while the cavedwellers have been investigated especially at Carniola and in the American caves.

A number of species of Cyclopidae and Cypridae, many of which are blind and colourless, have been brought up in well-water. The Amphipod Niphargus puteanus has long been known from a similar source in England and all over Europe, and several other blind Gammarids inhabit the subterranean waters and caves in various parts of the world. Among Isopods, Asellus cavaticus is recorded from wells and caves in various parts of Europe, Caecidotea stygia and C. nickajackensis from the Mammoth and Nickajack Caves in America, and two very remarkable blind Isopods are described by Chilton from the subterranean waters of New Zealand, viz. Cruregens fontanus, whose nearest allies are the marine Anthuridae, and the Isopods Phreatoicus typicus and P. assimilis, which bear an extraordinary resemblance superficially to Amphipods. Besides these, a small number of subterranean Decapoda are known which retain the eye-stalks but are without functional ommatidia. These are Troglocaris schmidtii, in Hungary, related to the fresh-water Atyid Xiphocaris of East Indian and East Asiatic fresh waters rather than to the South European Atyephyra; Palaemonetes antrorum, from artesian wells in Texas; and several species of Cambarus from the Eastern United States. A blind species of Cambarus, C. stygius, has been described from the caves of Carniola, and if this determination is correct, is the sole Cambarus occurring outside America.

It will be seen from the above account that the subterranean Crustacea are an exceedingly interesting and, in many respects, archaic group, many of which have survived in these isolated and probably uncompetitive districts, while many secular changes were going on in the quick world overhead.

The remaining fresh water Malacostraca may be mentioned under the headings of the groups to which they belong.

Only one “Schizopod,” apart from Paranaspides, is known from fresh-water lakes, viz. Mysis relicta, which was discovered in 1861 by Lovén in the Scandinavian lakes, and has since been found in the Finnish lakes, the Caspian Sea, Lake Michigan, and other localities in N. America, and Lough Erne in Ireland. This species is closely related to Mysis oculata of Greenlandic seas.

In the Southern Hemisphere we have a species of Anaspides, A. tasmaniae, occurring in mountain streams and tarns in Tasmania, a related form which haunts the littoral zone of the Great Lake in Tasmania, and a small species, Koonunga cursor, occurs in a little stream near Melbourne.

Of the Isopoda certain genera, viz. Asellus and Monolistra, are confined to fresh water, others, such as Sphaeroma, Idothea, Alitropus, and Cymothoa, have occasional fresh-water representatives. Packard describes a remarkable blind Isopod, Caecidotea, from the Mammoth Cave of Kentucky, which occupies a very isolated position, and in the same work gives a very complete exposition of the cave-fauna of North America and Europe.

The Phreatoicidae are a curious family of Isopods confined to the fresh waters of Australia and New Zealand, which bear a remarkable resemblance to Amphipods, being laterally compressed and possessing a subchelate hand on the anterior thoracic leg. These Isopods are exceedingly common in small mountain pools and in streams in Tasmania, and in the Great Lake in that country I have recently found a number of species which, together with some species of Amphipods, make up the dominant feature in the Crustacean fauna. One of these species may grow to fully an inch in length. The family is confined to the temperate regions, and is usually found on mountains. A number of species are known from the mainland of Australia, one coming from a high elevation on Mount Kosciusko, and another (Phreatoicopsis) from the forests of Gippsland attaining a great size, and living among damp leaves, etc.

The fresh-water Amphipoda all belong to the families Talitridae, Gammaridae, and Haustoriidae (see p. ).

Among the Talitridae, or Sand-hoppers, Orchestia and Talitrus have marine as well as fresh-water and land representatives, while the American Hyalella is entirely from fresh water, most of the species being peculiar to Lake Titicaca. Many of these animals are partly emancipated from an aquatic life. Thus Orchestia gammarellus, which is common on the sea-shore of the Mediterranean, frequently penetrates far inland, and was found in large numbers by Kotschy near a spring 4000 feet up on Mount Olympus.

Talitrus sylvaticus is very common among fallen leaves and decaying timber in Tasmania and Southern Australia, many miles from the sea, and often at an elevation of several thousand feet.

Among the Gammaridae, certain genera, e.g. Macrohectopus (Constantia), from Lake Baikal, are purely fresh-water. An enormous development of Gammaridae was discovered by Dybowsky in Lake Baikal, comprising 116 species, and lately a number more have been found by Korotneff. The majority of these were originally placed in the genus Gammarus, but Stebbing has rightly created a number of peculiar genera for them. Certain species are, however, placed in more widely distributed genera, e.g. Gammarus and Carinogammarus, which is also represented in the Caspian Sea. Korotneff found some remarkable transparent pelagic forms (Constantia) swimming in the abyssal regions at about 600 metres depth, the majority of them being blind, but some possessing rudimentary eyes, often on one side only.

Besides various species of Gammarus, a number of other Gammaridae are frequently found in brackish water. Among Haustoriidae Pontoporeia has representatives in both the oceans and inland lakes of the northern hemispheres (see p. ).

Of the Decapoda, seven families are typically fresh-water in habitat—the Aegleidae, containing the single species Aeglea laevis, related to the Galatheidae, which inhabits streams in temperate S. America; the Atyidae, a family of Prawns from the tropical rivers and lakes of the New and Old World, and in the Mediterranean region. A number of Palaemonidae are found in fresh water, e.g. Palaemonetes varians in Europe and N. America, while several species of Palaemon occur in lakes, streams, and estuaries of the tropical Old and New World.

The expeditions of Moore and Cunnington to Lake Tanganyika brought back an exceedingly rich collection of Prawns, comprising twelve species, all of which are peculiar to the lake, and this is all the more surprising since Lakes Nyasa and Victoria Nyanza are only known to contain one species, Caridina nilotica, which ranges all over Africa and into Queensland and New Caledonia. The Tanganyika species, however, all belong to purely fresh-water genera, and do not afford any suggestion that they are part of a relict marine fauna. It would appear that they have been differentiated in the lake itself during a long period of isolation.

Two groups of Brachyura, viz. the Thelphusidae and the Sesarminae (a sub-family of the Grapsidae), are fresh-water. Thelphusa fluviatilis is an inhabitant of North Africa, and penetrates into the temperate regions of the Mediterranean, and is said to be exceedingly common in the Alban Lake near Rome. Both these families have representatives on land, e.g. Potamocarcinus in Central and South America, and certain species of Sesarma, and the closely related Gecarcinidae of the West Indies.

The remaining families to be dealt with are the two Crayfish families—the Astacidae and the Parastacidae—which live in rapidly moving rivers and streams, and occasionally in lakes. A few species of both families have taken to a subterranean mode of life, and excavate burrows in the earth, e.g. the Tasmanian Crayfish, Engaeus fossor. The distribution of the Crayfishes has long engaged the attention of naturalists. It was first seriously studied by Huxley, and has subsequently been followed up, especially in North America, by Faxon and Ortmann, but our knowledge of the South American and Australian forms is still very incomplete. The Astacidae inhabit the northern temperate hemisphere, the Parastacidae the southern temperate hemisphere, the tropical belt being practically destitute of Crayfishes. Of the Astacidae the genus Astacus (Potamobius), including the common Crayfishes of Europe, occurs in Europe and in North America west of the Rockies. The genus Cambaroides, which in certain respects approaches Cambarus, is found in Japan and Eastern Asia. The very large genus Cambarus, on the other hand, only occurs in North America east of the Rockies, so that Cambaroides occupies a very isolated position. The occurrence of a Cambarus, C. stygius, in the caves of Carniola, has been recorded by Joseph, so that it would appear that this genus had a much wider range formerly than now.

Of the southern temperate Parastacidae, Australia and Tasmania have the genera Astacopsis and Engaeus; New Zealand has Paranephrops, South America Parastacus, and Madagascar Astacoides. The last named genus is rather isolated in its characters, possessing a truncated rostrum and a highly modified branchial system, but it agrees with all the other Parastacine genera, and differs from the Astacidae in the absence of sexual appendages on the first abdominal segment, and in the absence of a distinct lamina on the podobranchiae. The largest crayfish in the world is Astacopsis franklinii, found in quite small streams on the north and west coast of Tasmania. Specimens have been caught weighing eight or nine pounds, and rivalling the European Lobster in size. Crayfishes appear to be entirely absent from Africa.

It seems reasonable to suppose that the two families of Crayfishes characteristic respectively of the northern and southern hemispheres have been independently derived from marine ancestors, which have subsequently become extinct. Their complete absence in the tropics is striking, and Huxley drew attention to the fact that it is exactly in those regions where the Crayfishes are absent that the other large fresh-water Malacostraca are particularly well developed, and vice versa. Thus the large fresh-water Prawns are typically circumtropical in distribution, while the South African rivers abound with River-crabs, which, in general, are found wherever Crayfishes do not occur.

A few of the more interesting features in connection with the distribution of fresh-water Crustacea have now been touched upon. With regard to the origin of this fauna, we can see that a number of the species are comparatively recent immigrants from the sea, working their way up the estuaries of rivers, a proceeding which can be observed to be taking place to-day in a district like the Broads of Norfolk. Others, again, but these are few, appear to be true relict marine animals left stranded in arms of the sea that have been cut off from the main ocean, and have been gradually converted into fresh-water lakes and seas. Such are, perhaps, Mysis relicta and the rich Gammarid fauna of Lake Baikal, a lake that, in the presence of Seals, Sponges, and other marine forms, has clearly retained some of the characters of the ocean from which it was derived. The majority of the fresh-water species, however, have probably been evolved in situ, and their origin from marine ancestors is lost in an obscure past. The Crustacean fauna of the Caspian Sea shows us in an interesting manner the effects of isolation and changes in salinity, etc., on the inhabitants of a basin which once formed part of the ocean. The waters of the Caspian Sea are not fresh, but they are on the average about one-third as salt as that of the open ocean. The Crustacea, described by Sars, belong to undoubtedly marine groups, e.g. the Mysidae, Cumacea, and Amphipoda Crevettina, but the remarkable feature of these Caspian Crustacea is the great variety of peculiar species representing marine genera which are very poorly represented in the sea, thus indicating that the variety of the fauna is not due to a great variety of species having been shut up in the Caspian Sea to begin with, but rather that, after the separation from the sea, the isolated species began to vary and branch out in the most luxuriant way—whether from lack of competition or owing to the changing conditions of salinity it is difficult to say. As an example, the Cumacea of the Caspian Sea are ten in number, all belonging to peculiar genera related to Pseudocuma, except one species which is included in that genus. These Caspian forms make up the Family Pseudocumidae, which contains in addition only two marine forms of the genus Pseudocuma (see p. ). A very similar condition is found in the numerous Amphipods of the Caspian Sea. Considering the enormous changes that must have taken place in the distribution of land and water even during Tertiary times, it is astonishing that the fresh waters of the world do not contain more species in common with the ocean, but it must be considered that the limited area and comparatively uniform conditions of fresh-water lakes and streams would only permit a limited number of these forms to survive which could most easily adapt themselves to the changed conditions. And these would in all probability be the littoral species that were in the habit of passing up into the brackish waters of estuaries and lagoons, so that the uniform and limited nature of the fresh-water fauna can be accounted for to a certain extent by this hypothesis.

We have seen in dealing with the marine Crustacea of the littoral zone that the chief condition determining their distribution is temperature, and that the world may be divided into three chief areas of distribution for these animals, viz. the north temperate hemisphere, the tropics, and the south temperate hemisphere. It seems that the same division holds good for fresh-water Crustacea. We have already seen that the Crayfishes follow this rule, being practically absent from the tropics, and represented in the two temperate hemispheres by two distinct families, the Astacidae in the north and the Parastacidae in the south. Characteristic of the tropical belt are the absence of Crayfishes and the great development of Prawns and River-crabs. In the case of Entomostraca the great majority of the genera are cosmopolitan, especially those which live in small bodies of water liable to dry up, because these forms always have special means of dissemination in the shape of resting eggs which can be transported in a dry state by water-birds and other agencies to great distances; but those genera which inhabit large lakes are more confined in their distribution. The Copepod genus Diaptomus, characteristic of lake-plankton, ranges all over the northern hemisphere and into the tropics, but it is almost entirely replaced in the southern hemisphere by the related but distinct genus Boeckella, which occurs in temperate South America, New Zealand, and southern Australia, and was found by the author to be the chief inhabitant in the highland lakes and tarns of Tasmania, Diaptomus being entirely absent. Of the Cladocera there are a number of pelagic genera (e.g. Leptodora, Holopedium, Bythotrephes) entirely confined to the lakes of the northern hemisphere. The distribution of Bosmina is interesting. This genus is distributed all over the north temperate hemisphere in lakes and ponds of considerable size, not liable to desiccation; in the New World it passes right through the tropics into Patagonia, the chain of the Andes doubtless permitting its migration. In the tropics of the Old World it is unknown, but it turns up again, as the author recently found, as a common constituent in the plankton of the Tasmanian lakes. There is another instance of a group of Crustacea, characteristic of the north temperate hemisphere, being entirely absent from the tropics, at any rate of the Old World, but reappearing in the temperate regions of Australasia. The commonest fresh-water Amphipods in this region belong to the genus Neoniphargus, intermediate in its characters between the northern Niphargus and Gammarus, but grading almost completely into the latter. Both Niphargus and Gammarus are absolutely unknown from the tropics, but whether, like Bosmina, they occur in the Andes and temperate South America is not known; it seems, however, probable that they have reached Southern Australia by way of South America rather than through the tropics of Asia and Australia, where there is no range of mountains to permit the migration of a group of animals apparently dependent on a temperate climate. The other common fresh-water Amphipod in temperate Australia and New Zealand is Chiltonia, whose nearest ally is Hyalella from Lake Titicaca on the Andes, and temperate South America.

The Anaspidacea and Phreatoicidae, which are so characteristic of temperate Australia, and are generally of an Alpine habit, have never been found in South America, but the Anaspidacea are represented by numerous marine forms in the Permian and Carboniferous strata of the northern hemisphere, so that it is probable that this group reached the southern hemisphere from the north through America.

The distribution of the fresh-water Crustacea, therefore, in the temperate southern hemisphere affords strong evidence in favour of the view that the chief land-masses of this hemisphere, which are at present separated by such vast stretches of deep ocean, were at no very remote epoch connected in such a way as to permit of an intermixture of the temperate fauna of New Zealand, Australia, and South America. While this connexion existed, a certain number of forms characteristic of the northern hemisphere, which had worked through the tropics by means of the Andes, were enabled to reach temperate Australia and New Zealand. The existence of a coast-line connecting the various isolated parts of the southern hemisphere would, of course, also account for the community which exists between their littoral marine fauna. It is impossible to enter here into the nature of this land-connexion which is becoming more and more a necessary hypothesis for the student of geographical distribution, whatever group of animals he may choose, but it may be remarked that the connexion was probably by means of rays of land passing up from an Antarctic continent to join the southernmost projections of Tierra del Fuego, Tasmania, and New Zealand.