Chapter VI (2)

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. 119), 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. 119) 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.

FIG. 119.—_Birgus latro_, ♀, × ⅙, ventral view. _Ab_, First pleopod;
_T_, last pereiopod.
]

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.

FIG. 120.—Dorsal view of abdomen, =A=, of _Cenobita_, sp.; =B=, of
_Birgus latro_. _T_, Telson; 1–6, 1st–6th abdominal segments.
]

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. 120, 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. 120, 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. 121), 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.[144]

FIG. 121.—_Lithodes maia_, ♀, in ventral view, × ¼. The abdomen is
flexed on the thorax, so that its dorsal surface is seen. _l.3_,
Lateral plates of third abdominal segment; _l.5_, left lateral plate
of fifth abdominal segment; _m_, marginal plate; _T_, brush-like
last pereiopod; _Te.6_, telson and sixth abdominal segment.
]

In some of the Eupagurinae, e.g. _Pylopagurus_, feebly calcified plates are present upon the segments of the abdomen (Fig. 122, A).

In the most primitive of the Lithodidae we witness the reduction (Fig. 122, 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.

FIG. 122.—Diagrams of abdomen: =A=, of _Pylopagurus_, sp.; =B=, of
_Hapalogaster cavicauda_; =C=, of _Dermaturus hispidus_; =D=, of
_Lithodes maia_, ♂; =E=, of _Acantholithus hystrix_. _c_, Central
plates; _l_, lateral plates; _m_, marginal plates; _T_, telson; 1–6,
1st–6th abdominal segments. (After Bouvier.)
]

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. 122, 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_,[145] which appear to be Pagurids that have deserted their shells.

FIG. 123.—Four stages in the development of _Eupagurus longicarpus_ or
_E. annulipes_, × 20. =A=, Ventral view of Zoaea; =B=, lateral view
of Metazoaea; =C=, dorsal view of Glaucothoe; =D=, dorsal view of
adolescent stage. _Ab.6_, 6th abdominal appendage; _Mxp.1_, _Mxp.3_,
1st and 3rd maxillipedes. (After M. T. Thompson.)
]

The metamorphosis of the Hermit-crabs has recently been studied by M. T. Thompson.[146]

The Zoaea (Fig. 123, 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. 123, C), which corresponds to the Megalopa of Brachyura (Fig. 125, 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. 123, 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. 118, 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. 119) 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. 121) 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.=[147]

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.

FIG. 124.—=A=, Zoaea, × 24, and =B=, Metazoaea, × 13, of _Corystes
cassivelaunus_. _Ab_, 3rd abdominal segment; _An_, 1st antenna; _E_,
eye; _G_, gills; _M_, 1st maxillipede; _T.8_, last thoracic
appendage. (After Gurney.)
]

FIG. 125.—Later stage (Megalopa) in the development of _Corystes
cassivelaunus_, × 10. _A_, Antenna; _Ab_, 3rd abdominal segment;
_C_, great chela; _T.8_, last thoracic appendage. (After Gurney.)
]

The larva hatches out as a Zoaea[148] (Fig. 124, 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. 124, 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. 125), 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. 125).

The Brachyura must be considered under the following subdivisions:—

=Tribe 1. Dromiacea.=

All authorities are agreed that these[149] 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,[150] 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.

FIG. 126.—_Dromia vulgaris_, × 1. (After Milne Edwards and Bouvier.)
]

=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. 126) 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_,[151] 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_[152] presents the same peculiarity.

FIG. 127.—_Cymonomus granulatus_, × 1. _A.1_, _A.2_, 1st and 2nd
antennae; _E_, eye-stalk; _S_, extra-orbital spine of carapace.
(After Lankester.)
]

_Cymonomus granulatus_ (Fig. 127) 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.

FIG. 128.—_Calappa granulata_, from in front, × ½. _C_, Hand of
chelipede; _T_, walking legs. (After Garstang.)
]

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_.[153] 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. 128, 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. 128 by the two black cavities below the snout in the middle line.

A similar method is pursued by the related _Matuta banksii_[153] (Fig. 129), 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. 131) 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.

FIG. 129.—Dorsal view of _Matuta banksii_, × 1. (From an original
drawing prepared for Professor Weldon.)
]

In the Leucosiidae, of which the Mediterranean _Ilia nucleus_ (Fig. 130) 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. 128) circumtropical, and extending into the warmer temperate seas. _Matuta_ (Fig. 129) from the Indo-Pacific.

FIG. 130.—Dorsal view of _Ilia nucleus_, × 1. (From an original
drawing prepared for Professor Weldon.)
]

=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. 130) 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. 127) 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[154] 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. 125, 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[155] 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.

FIG. 131.—Dorsal view of _Lupa hastata_, × 1. (From an original
drawing prepared for Professor Weldon.)
]

=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. 131).

=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. 133), 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[156] 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. 132), 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.

FIG. 132.—_Encephaloides armstrongi_, × 1. The long walking legs are
omitted. _C_, Great chela; _G_, one of the greatly swollen
gill-chambers. (After Alcock.)
]

=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. 132).

=Fam. 2. Parthenopidae.=—The chelipedes are much larger than the other legs. Orbits complete. _Lambrus_ (Fig. 133), _Parthenope_.

FIG. 133.—_Lambrus miersi_, × 1. (After Milne Edwards and Bouvier.)
]

=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. 134) 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.

FIG. 134.—Dorsal view of _Pachygrapsus marmoratus_, × ⅓. (From an
original drawing prepared for Professor Weldon.)
]

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.

FIG. 135.—_Gelasimus annulipes_, × 1. =A=, Female; =B=, male. (After
Alcock.)
]

_Gelasimus_ (Fig. 135) 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.[157]

=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.=[158]—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. 134), 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. 135), in the tropics of both hemispheres. _Ocypoda_, with similar distribution.