CHAPTER VI. Crustacea (_continued_)—Eumalacostraca (_continued_): Eucarida—

CRUSTACEA (CONTINUED)—EUMALACOSTRACA (CONTINUED): EUCARIDA— EUPHAUSIACEA—COMPOUND EYES—DECAPODA

DIVISION 4. EUCARIDA.

The carapace fuses with all the thoracic segments. The eyes are pedunculate. The mandible is without a lacinia mobilis. There are no oostegites, the eggs being attached to the endopodites of the pleopods. The hepatic caeca are much ramified, the heart is abbreviated and saccular, the spermatozoa are spherical with radiating pseudopodia, and development is typically attended by a complicated larval metamorphosis.

=Order I. Euphausiacea.=

FIG. 101.—Calyptopis larva of Euphausia pellucida, × about 20. A.1, 1st antenna; Ab.6, 6th abdominal segment; E, eye; M, maxillipede. (After Sars.) ]

The =Euphausiidae= agree with the Decapoda in passing through a complicated larval metamorphosis. The young hatch out as Nauplii, with uniramous first antennae and biramous second antennae and mandibles. In the next stage, or “Calyptopis” (Fig. 101), which corresponds exactly to the Zoaea of the Decapoda, two pairs of maxillae and a pair of biramous maxillipedes are added; the hinder thoracic segments are undifferentiated, but the abdomen is fully segmented, and the rudiments of the sixth pair of pleopods are already visible.

In the next stage (“Furcilia”) the other abdominal pleopods are added, the whole series being completed before the thoracic appendages number more than two or three. This stage corresponds to the Metazoaea of the Decapoda, and the interference in the orderly differentiation of the segments with their appendages from before backwards is a phenomenon which we shall meet again when we treat of Decapod metamorphosis. It is evidently a secondary modification, furnishing the larva precociously with its most important swimming organs so as to enable it to lead a pelagic existence. The frequent violation of the law of metameric segmentation, that the most anterior segments being the first formed should be the first to be fully differentiated, leads us to suppose that the larval stages of the Eucarida at any rate do not represent phylogenetic adult stages through which the Malacostraca have passed. Nor do they, perhaps, even represent primitive larval stages, but have been secondarily acquired from an embryonic condition which used to be passed through within the egg-membranes, as in Nebalia and the Mysidacea, when the order of differentiation of the segments was normal. The case is a little different with the Nauplius larva. This larval form, in an identical condition, is found both in the Entomostraca as a general rule, and again in certain Malacostraca, viz. the Euphausiidae and the Peneidea. Whatever its phylogenetic meaning may be, we may be quite certain that the ancestor of the two great divisions of the Crustacea had a free-swimming Nauplius larva, and this conclusion is confirmed by the probable presence of a Nauplius larva in Trilobites.

The Euphausiidae, in contradistinction to the Mysidae, are frequently met with in the surface-plankton. Euphausia pellucida (Fig. 102) is of universal distribution, and is frequently taken at the surface as well as at considerable depths.

Many noteworthy features in Euphausiid organisation are brought out in Fig. 102. The shrimp-like appearance of the carapace and antennae indicate the special Decapodan affinities of the family; noteworthy, also, are the single series of gills and the biramous thoracic and abdominal limbs, similar to those of the Mysidacea. The Euphausiidae also possess phosphorescent organs of a highly developed kind, and these are usually situated, as in the type figured, upon the outer margins of the stalked eyes, on the bases of the second and seventh thoracic limbs, and on the ventral median line on the first four abdominal segments. These organs are lantern-like structures provided with a lens, a reflector, and a light-producing tissue, and they are under the control of the nervous system. Their exact use is not known, any more than is the use of phosphorescence in the majority of organisms which produce it; but in certain cases it appears that the Euphausiids make use of their phosphorescent organs as bull’s eye lanterns for illuminating the dark regions into which they penetrate or in which some of them permanently dwell. At any rate, associated with the presence of these organs in some deep-sea Euphausiids are remarkable modifications of the eyes; and we may perhaps here fittingly introduce a short discussion of these visual modifications in deep-sea Crustacea, and the conditions which call them forth.

FIG. 102.—Euphausia pellucida, female, × 5. G, Last gill; L, luminous organ of first leg; L′, luminous organ of 2nd abdominal segment; T, biramous thoracic appendages. (After Sars.) ]

FIG. 103.—=A=, Sections (diagrammatic) of Crustacean compound eye, =A=, with pigment in light-position for mosaic vision; =B=, with pigment in dark-position for refractive vision. c, Corneal lens; c.g, corneagen cells; cr, crystalline cone; f, basal membrane, or membrana fenestrata; ip, irido-pigment; n, nerve; r, retinula; rh, rhabdom; rp, retino-pigment; v, vitrella. ]

The =compound eyes of Crustacea= resemble those of Insects in that they are composed of a very large number of similar elements or “ommatidia,” more or less isolated from one another by pigment. Each ommatidium consists typically of a corneal lens (Fig. 103, c), secreted by flat corneagen cells (c.g) below; beneath the corneal lens is a transparent refractive body called the “crystalline cone” (cr), which is produced by a number of cells surrounding it called the “vitrellae” (v). Below the crystalline cone comes the “rhabdom” (rh), produced and nourished by “retinulacells” (r). The rhabdom is a transversely striated rod, constituting the true sensory part of each ommatidium, and is in connexion at its lower end with a nerve-fibre (n), passing to the optic ganglion. The rhabdoms rest upon a membrane (f) called the “membrana fenestrata.” Each ommatidium is isolated from its fellows which surround it by a complete cylinder of pigment, part of which is especially crowded round the crystalline cone, and is known as “irido-pigment” (ip), while the part which surrounds the rhabdom is called “retino-pigment” (rp).

When the pigment is arranged in this way, as in Fig. A, only those rays of light which strike an ommatidium approximately at right angles to the corneal surface can be perceived, since only these can reach the top of the rhabdom; the others pass through the crystalline cones obliquely, and are absorbed by the cylinder of pigment surrounding each ommatidium, so that they neither reach the rhabdom of the ommatidium which they originally entered, nor can they penetrate to the rhabdom of neighbouring ommatidia. This gives rise to what is known as “mosaic vision,” that is to say, each ommatidium only perceives the rays of light which are parallel to its long axis, and in this way an image is built up of which the various points are perceived side by side by means of separate eye-elements. The distinctness and efficiency of this mode of vision depends chiefly upon the number of ommatidia present, and the completeness with which they are isolated from one another by the pigment. Now this form of vision, depending as it does upon the absorption of a great number of the light-rays by pigment, and the transmission of only a limited number to the sensory surface, is only possible when there is a strong light, and there is no need for economising the light-rays. The most important discovery was made by Exner, that the majority of animals with compound eyes had the power of so arranging the pigment in their eyes as to enable them to see in two ways. In bright light the pigment is situated as in Fig. 103, A, so as completely to isolate the rhabdoms from one another (day-position); but in the dusk the pigment actively migrates, the irido-pigment passing to the surface (B) near the tops of the crystalline cones, and the retino-pigment passing interiorly to rest on the membrana fenestrata at the bases of the rhabdoms (night-position). When this happens the rays of light which strike the ommatidia at all sorts of angles, instead of being largely absorbed by the pigment, are refracted by the crystalline cones and distributed over the tops of the rhabdoms, passing freely from one ommatidium to another. In this way the eye acts on this occasion, not by mosaic vision, but on the principle of refraction, as in the Vertebrate eye. Of course the distinctness of vision is lost, but an immense economy in the use of light-rays is effected, and the creature can perceive objects and movements dimly in the dusk which by mosaic vision it could not see at all. The pigment is contained in living cells or chromatophores, and it is carried about by the active amoeboid movements of these cells with great rapidity.

Now, besides the active adaptability to different degrees of light brought about in the individual by these means, we find Crustacea living under special conditions in which the eyes are permanently modified for seeing in the dusk, and this naturally occurs in many deep-sea forms.

Doflein has examined the eyes of a great number of deep-sea Brachyura dredged by the Valdivia Expedition, and as the result of this investigation he states that the eyes of deep-sea Brachyura are never composed of so many ommatidia, nor are they so deeply pigmented as those of littoral or shallow water forms. At the same time an immense range of variation occurs among deep-sea forms which are apparently subjected to similar conditions of darkness, a variation stretching from almost normal eyes to their complete degeneration and the fusion of the eye-stalks with the carapace; and this variation is very difficult to account for. A very frequent condition for crabs living at about 100 fathoms, and even more, is for either the irido-pigment or the retino-pigment to be absent, for the number of ommatidia to be reduced, and for the corneal lenses to be greatly arched. There can be little doubt that these crabs use their eyes, not for mosaic vision, but to obtain the superposition-image characteristic of the Vertebrate eye. In deeper waters, where no daylight penetrates at all, this type of eye is also met with, and also further stages in degeneration where all pigment is absent, and the ommatidia show further signs of reduction and degeneration, e.g. Cyclodorippe dromioides. In a few forms, e.g. Cymonomus granulatus among Brachyura, and numerous Macrura, the ommatidia may entirely disappear, and the eye-stalks may become fused with the carapace or converted into tactile organs.

Progressive stages in degeneration, correlated with the depth in which the animals are found, are afforded by closely related species, or even by individuals of apparently the same species. Thus in the large Serolidae of Antarctic seas, Serolis schytei occurs in 7–128 metres, and has well-developed eyes; S. bronleyana, from 730 to 3600 metres, has small and semi-degenerate eyes; while S. antarctica in 730–2920 metres is completely blind. Lispognathus thompsoni is a deep-water spider-crab, and the individuals taken at various depths are said to exhibit progressive stages in degeneration according to the depth from which they come.

At the same time many anomalies occur which are difficult to explain. In the middle depths, i.e. at about 100 fathoms, side by side with species which have semi-degenerate or, at any rate, poorly pigmented eyes, occur species with intensely pigmented eyes composed of very numerous ommatidia, e.g. the Galatheid Munidopsis and several shrimps, while in the true abysses many of the species have quite normal pigmented eyes. This is especially the case with the deep-sea Pagurids, of which Alcock describes only one species, Parapylocheles scorpio, as having poorly pigmented eyes. An attempt to account for this was made by Milne Edwards and Bouvier, who pointed out that the truly deep-sea forms with well-developed eyes were always Crustacea of a roving habit, which were perhaps capable of penetrating into better lit regions, and to whom well-developed eyes might be useful, while the degenerate forms were sluggish. This explanation cannot be held to account for the phenomenon, as too many deep-sea forms with fairly normal eyes are known which are never taken outside deep waters. Doflein (loc. cit.) points out that in the Brachyura of the deep sea there is a remarkable correlation between the degree of degeneration of the eye and the size of the eggs—the large-egged forms having unpigmented and degenerate eyes, while the species with small eggs have pigmented eyes. He supposes that the species with large eggs undergo a direct development without pelagic free-swimming larvae, and that since they never reach the surface their eyes never meet with the necessary stimulus of light for the development of pigment; whereas the small-egged species undergo a pelagic larval existence when this stimulus is present and gives the necessary initiative for the development of the pigment.

Another factor enters into the question of eye-degeneration in the Crustacea. The great majority of deep-sea animals, including many deep-sea Crustacea, are phosphorescent, and it is certain that although daylight never penetrates into the abysses of the ocean, yet there is considerable illumination derived from the phosphorescence of the inhabitants of these regions.

Alcock points out in this connexion that the Pagurids, which are conspicuous in the great depths as animals with normally developed eyes, carry about anemones with them, and these organisms are very frequently phosphorescent to a high degree. It may well be, therefore, that the Pagurids are enabled to use their eyes in the normal manner owing to the phosphorescent light which they carry about with them, and this use of phosphorescent light may apply to a number of deep-sea Crustacea whose eyes are not at all or only partially degenerate.

An extremely interesting case of the use of phosphorescent light is given by Chun. In a number of Euphausiids occurring in deep waters each compound eye is divided into two parts—a frontal and ventro-lateral—which differ from one another very greatly in the nature and disposition of their ommatidia.

FIG. 104.—Section of eye of Stylocheiron mastigophorum. =A=, Frontal portion; =B=, ventro-lateral portion; =C=, phosphorescent organ; =D=, entrance of optic nerve; c, corneal lens; cr, crystalline cone; pg, pigment; ret, retinula; rh, rhabdom. (After Chun.) ]

In the frontal portion (Fig. 104, A) the ommatidia are few in number and long, the corneal lenses are highly arched, and the pigment is reduced to a few clumps in the iris. This part of the eye is evidently adapted for forming a vague superposition-image in the dusk. The ventro-lateral part (B), on the other hand, is composed of numerous small ommatidia, the crystalline cones of which can be completely isolated from one another by the irido-pigment. Immediately below this part of the eye is a phosphorescent organ (C) provided with a lens and tapetum. Chun suggests that the ventro-lateral part of the eye is used for obtaining a clear mosaic image of objects illuminated by the phosphorescent organ, while the frontal part of the eye is used for obtaining general visual impressions in dimly lit regions. This curious differentiation of the eye into two parts apparently only occurs in predaceous animals, which capture their prey alive upon the bottom, and to whom a clear vision of moving organisms is a necessity.

Another instance of Crustaceans making use of their own light is given by Alcock, who found two deep-sea prawns, Heterocarpus alphonsi and Aristaeus coruscans, at about 500 fathoms in the Indian Ocean. These animals produce a highly phosphorescent substance which they eject from the antennary glands, and they possess very large, deeply-pigmented eyes.

The whole subject of the modification of the pigment and structure of Crustacean eyes is an interesting one, because it presents us with one of those cases in which the direct response to a stimulus acting within the lifetime of the individual seems to run parallel to the fixed adaptations of a whole species, which have become hereditary and apparently independent of the external stimulus of light or of the absence of light. As far as is known, however, the direct response of the individual to the absence of light is limited to the reduction or disappearance of the pigment, and does not extend to those structural changes in the ommatidia which are characteristic of so many deep-sea forms.

=Order II. Decapoda.=

The Decapoda, together with the Euphausiidae, make up the Division Eucarida, the members of which differ from the Orders hitherto described in a number of characters, e.g. the presence of a carapace covering the whole of the thorax, the absence of a brood-pouch formed of oostegites, the presence of a short heart, of spermatozoa with radiating pseudopodia, and of a complicated larval metamorphosis, of which the Zoaea stages are most prominent.

The Decapoda differ from the Euphausiidae chiefly in the anterior three thoracic limbs being turned forwards towards the mouth to act as maxillipedes, and in the five succeeding thoracic limbs being nearly always uniramous and ambulatory or chelate; there are typically present three serial rows of gills attached to the thoracic segments, an upper series (“pleurobranchiae”) attached to the body-wall above the articulation of the limbs, a middle series (“arthrobranchiae”) attached at the articulation of the limbs, and a lower series (“podobranchiae”) attached to the basal joints of the limbs. These gills are enclosed in a special branchial chamber on each side of the thorax, formed by lateral wings of the carapace known as “branchiostegites.” The gills of each series are never all present in the same animal, the anterior and posterior members showing a special tendency to be reduced and to disappear. In this manner “branchial formulae” can be constructed for the various kinds of Decapods, which differ from the ideal formula in a manner distinctive of each kind. The second maxilla is always provided with an oar-like appendage on its outer margin (exopodite), known as the “scaphognathite,” which, by its rhythmical movement, keeps up a constant current of water through the gill-chamber.

A complicated auditory organ is present on the basal joint of the first antennae; this is a sac communicating with the exterior and lined internally with sensory hairs. The animal is said to place small pieces of sand, etc., in its ears to act as otoliths. Anaspides (see p. 116) is the only other Crustacean which has an auditory organ in this position.

The larval histories of the Decapods are of great interest, and will be given under the headings of the various groups. The first discoverer of the metamorphosis of the Decapoda was the Irish naturalist J. V. Thompson, certainly one of the ablest of British zoologists. In 1828, in his Zoological Researches, he describes certain Zoaeas of the Brachyura and proves that these animals are not an adult genus, as supposed, but larval forms. But Rathke, in 1829, described the direct development of the Crayfish; and Westwood, after describing the direct development of Gecarcinus, utterly denied Thompson’s assertions concerning metamorphosis. Thompson replied in the Royal Society Transactions for 1835, and described the Megalopa stage of Cancer pagurus. Rathke, although previously an opponent of Thompson, subsequently made confirmatory observations upon the larvae of the Anomura; and Spence Bate clinched the matter by describing Brachyuran metamorphosis with great accuracy in the Philosophical Transactions for 1859. Since then a mass of work has been done on the subject, though much detail still remains to be elucidated.

The Decapoda fall into three sub-orders, which graduate into one another—(i.) the Macrura, including the Lobsters, Crayfishes, Shrimps, and Prawns; (ii.) the Anomura, including the Hermit-lobsters and Hermit-crabs; and (iii.) the Brachyura or true Crabs.

=Sub-Order 1. Macrura.=

This sub-order is characterised by the large abdomen, furnished with five pairs of biramous pleopods, and ending in a powerful tail-fan composed of the telson and the greatly expanded sixth pair of pleopods, the whole apparatus being locomotory. The second antennae are furnished with very large external scales, representing the exopodites of those appendages. Some of the Shrimps and Prawns closely resemble the “Schizopods,” but the pereiopods are nearly always uniramous. Several subdivisions of the Macrura are recognised.

=Tribe 1. Nephropsidea.=

This tribe includes the Lobsters and Crayfishes, animals well known from their serviceableness to man. There are three families, which will be treated separately.

=Fam. 1. Nephropsidae.= The podobranchs are not united with the epipodites, and the last thoracic segment is fixed and fused to the carapace. The chelae are generally asymmetrical. The most important Lobsters are the European and the American species—Homarus vulgaris (= Astacus gammarus) and H. americanus respectively; these animals engage a large number of people in the fisheries. It is estimated that in America about £150,000 are spent every year on Lobsters.

The genus Nephrops contains the small Norwegian lobster and other forms.

Herrick gives some interesting particulars with regard to the life-history of the American species. The largest recorded specimen weighed about twenty-five pounds, and measured twenty inches from rostrum to tail; similar European specimens have been recorded, but, on the average, they are not so large as the American forms.

The Lobster, like all Crustacea, undergoes a series of moults as the result of increase in size, shedding the whole of the external integument in one piece. This is accomplished by a split taking place on the dorsal surface at the junction of thorax and abdomen; through the slit so formed the Lobster retracts first his thorax with all the limbs, and then his abdomen. When first issuing from the old shell the animal’s integument is soft and pulpy, but the increase in size of the body is already manifest; this increase per moult, which is approximately the same in young and adult animals, varies from 13 to 15 per cent of the animal’s length. According to this computation, a Lobster 2 inches long has moulted fourteen times, 5 inches twenty times, and 10 inches twenty-five times, and it may be roughly estimated that a 10–inch Lobster is four years old. Young Lobsters probably moult twice a year, and so do adult males, but females only moult once a year soon after the young are hatched out.

The process of moulting or ecdysis is an exceedingly dangerous one to the Lobster and to Crustacea in general, and is very frequently fatal. There is, first of all, the danger of the act not being accomplished skilfully, when death always ensues. The Lobster remains soft and unprotected for about six weeks after the ecdysis, and is very apt to fall a prey to the predaceous fish, such as Sharks, Skates, Cod, etc., which feed upon it. There are, however, some peculiar adaptations connected with the process which are of interest. In order to facilitate the ecdysis, areas of absorption are formed upon the dorsal and ventral surfaces of the carapace, on the narrower parts of the chelipedes, and at other places; in these areas the calcium carbonate is absorbed, and the old shell becomes elastic and thin, so as to allow a more easy escape for the moulting Lobster. It has been noticed that while this is taking place large concretions of calcium carbonate are formed at the sides of the stomach, known as “gastroliths,” which perhaps represent the waste lime that has been abstracted from the areas of absorption. After moulting the Lobster is in great need of lime for stiffening his shell, and it has been noticed that on these occasions he is very greedy of this substance, even devouring his own cast-off skin.

The male Lobster is especially prized on account of his larger chelae, but in both sexes the chelipedes are differentiated into a smaller cutting pincer and a larger crushing one. Lobsters may be right or left handed, with the large crushing claw on the right or left hand, and sometimes specimens occur with the smaller cutting pincers on both chelipedes, and very rarely, indeed, with crushing claws on both sides. Crustacea very commonly have the power of casting off a limb if they are seized by it or if it is injured, and of regenerating a new one. In the Lobster a so-called breaking joint is situated on each leg at the suture between the fused second and third segments; a membrane being pushed inwards from the skin, which not only serves to form a weak joint where rupture may easily take place, but also to stop excessive bleeding after rupture. In the newly-hatched larvae there is a normal joint between the second and third segments; and autotomy, or the voluntary throwing away of a limb, never occurs until the fourth larval stage, when the breaking joint is formed. Autotomy is a reflex act under the control of the segmental ganglion; if a Crab or Lobster be anæsthetised, and then a limb be injured or broken off below the breaking joint, the animal forgets to throw the injured leg or stump off at the breaking joint, a proceeding which always occurs under normal conditions. The regeneration of a limb starts from a papilla which grows out of the breaking joint, and after a number of moults acquires the specific form of the limb that has been lost. A number of interesting observations have been made upon the regeneration of the limbs in Crustacea. It was in the Hermit-crab that Morgan proved that regeneration and the liability to injury do not always run parallel, as Weismann held they should, since the rudimentary posterior thoracic limbs, which are never injured in nature, can regenerate when artificially removed as easily as any others. Przibram has shown that in the shrimp Alpheus, whose chelipedes are highly asymmetrical, if the large one be cut off, the small one immediately begins to grow and to take on the form of the large one, while the regenerated limb is formed as the small variety. This remarkable inversion in the symmetry of the animal clearly ensures that, if the large chela is injured and thrown away, the least amount of time is wasted in providing the shrimp with a new large claw.

To return to the Lobster; for the majority of the individuals there is a definite breeding season, viz. July and August, but a certain proportion breed earlier or later. A female begins to “berry” at about eight inches in length, and to produce more and more eggs up to about eighteen inches, when as many as 160,000 eggs are produced at a time; after this there is a decline in numbers. A female normally breeds only once in two years. Strict laws are enforced forbidding the sale of Lobsters and Crabs “in berry” in both England and America. The period of incubation, during which the developing eggs are attached to the swimmerets of the female, lasts about ten or eleven months, so that the larvae are hatched out approximately in the following June. On hatching, the larva, which measures about one-third of an inch, and is in the Mysis stage (i.e. it possesses all the thoracic limbs in a biramous condition, but is without the abdominal limbs), swims at first on the surface. After five or six months of this life, during which the abdominal pleopods are added from before backwards, it sinks to the bottom, loses the exopodites of the thoracic limbs, and is converted into the young Lobster, measuring about half an inch in length. The little Lobster starts in deepish water, and gradually crawls towards the shore; here it passes its adolescence, but on coming to maturity it migrates out again into the deep water.

=Fam. 2. Astacidae.=—In this family, which includes all the European and North American Crayfishes, Astacus (Potamobius) and Cambarus, the podobranchs are united with the epipodites, the last thoracic segment is free, there is only one pleurobranch or none at all, the gills have a central lamina, but the filaments are without terminal hooks, and the endopodites of the first two pairs of abdominal appendages in the male serve as copulatory organs. For the distribution, etc., of these forms see p. 213.

=Fam. 3. Parastacidae.=—This family includes the Crayfishes of the Southern Hemisphere, viz. Parastacus from South America, Astacopsis and Engaeus from Australia, Paranephrops from New Zealand, and Astacoides from Madagascar. These genera agree with the Potamobiidae in the union of the podobranchs with the epipodites, and in the free condition of the last thoracic segment, but there are generally four pleurobranchs, the gills are without a lamina, the filaments have terminal hooks, and there are no sexual appendages in the male. For distribution, etc., see also p. 213.

The larval development in the Crayfishes is still more abbreviated than in the Lobsters, the Mysis stage being passed through within the egg-membranes. The young, when they hatch out, are furnished with hooks upon the chelipedes, by which they anchor themselves to the pleopods of the mother.

=Tribe 2. Eryonidea.=

FIG. 105.—Willemoesia inornata, × ⅓. (From a figure prepared for Professor Weldon.) ]

These are remarkably archaic animals of great rarity, though they were common enough in Triassic seas, and have come down to us as fossils from those times, being thus among the oldest Decapoda known. They only survive now as deep sea species, and the genus discovered by the Challenger, Willemoesia (Fig. 105), confirmed the expectations of the Challenger naturalists that the abysses of the ocean would contain relics from older periods which had managed to survive where the competition was not so keen. The genus Willemoesia is very widely distributed, being dredged up from below a thousand fathoms in the Indian Ocean, the Mediterranean, North and South Atlantic, and the Pacific oceans. All the walking legs are chelate, and the animal is quite blind, as are all the Eryonidea, the eye-stalks being fused with the carapace.

Only a single family =Eryonidae= is recognised.

=Tribe 3. Peneidea.—Tribe 4. Caridea.=

We will now consider the Shrimps and Prawns, since in them occurs the most complete metamorphosis found in the Decapoda. The Peneidea are distinguished from the ordinary Prawns and Shrimps (Caridea) by having the first three instead of the first two pereiopods chelate. The genus Peneus affords several species which are of commercial value as objects of food; the edible Prawns of the Mediterranean belong to this genus, while in the North Sea two of the Caridea, viz. the Shrimp, Crangon vulgaris, and the Prawn, Palaemon serratus, are the forms very commonly eaten. Both subdivisions are well represented in the deep sea fauna from all parts of the world. Glyphocrangon spinulosa (Fig. 110, p. 164) is a deep sea Shrimp with eyes that have lost their pigment, and with the body covered with spines, while the last abdominal segment is fused with the telson to form a sharp bayonet-like process at the hind end of the body. Some of the deep-sea Prawns of the Indian Ocean are described by Alcock as possessing peculiar secondary sexual characters. Thus Parapeneus rectacutus ♂ has one lash of the first pair of antennae peculiarly bent to form a clasping organ, while Aristaeus crassipes has a hook on the end of the third maxillipede. In the latter the females have much longer rostra than the males, and are in general more powerfully built, so that they seem to have usurped the proper functions of the male, and probably engage in combats with one another over his person.

FIG. 106.—Nauplius larva of Peneus, sp. × 25. (From Balfour, after F. Müller). ]

As a general rule the Shrimps and Prawns occur in large shoals in the shallow waters of the littoral zone, and they have a remarkable power of adapting their colours to the surroundings in which they happen to be at any particular moment. This is brought about by the variously coloured chromatophores, which contract and expand in obedience to a stimulus transmitted through the eyes of the animal. A number of the Palaemonidae go up rivers into fresh water, while one family, the Atyidae, live in the completely fresh water of rivers and inland lakes. The Peneidea undergo a very complete metamorphosis which is primitive in respect to the order of formation of the segments from before backwards. The larva hatches out as a Nauplius (Fig. 106), which by the orderly addition of segments behind is converted into the Protozoaea (Fig. 107), possessing two pairs of biramous maxillipedes. It should be noted that the maxillae, which are foliaceous in the adult, are laid down in this condition in the larva, and this principle holds good throughout Crustacean metamorphosis, viz. that when a limb is foliaceous in the adult it is foliaceous in the larva, and when biramous in the adult it is biramous in the larva. Whilst the rest of the thoracic limbs are still rudimentary, the sixth pair of pleopods are being precociously developed (Fig. 108), being the only precociously formed limbs in the Peneidea, though the abdominal segments are fully marked off before the thoracic segments, and so must be considered as precocious in development. When the biramous thoracic limbs are completed the abdominal biramous pleopods are added, beginning from in front backwards. Thus the Mysis stage (Fig. 109) is reached, which resembles in all particulars the adult condition of the Schizopoda. The adult Prawn develops from this stage by the loss of some or all of the exopodites on the thoracic pereiopods.

FIG. 107.—Protozoea larva of Peneus, sp. × 25. (From Balfour, after F. Müller.) ]

FIG. 108.—Zoaea larva of Peneus, sp. × 25. A, A′, 1st and 2nd antennae; Ab.6, 6th abdominal appendage; Mxp, 2nd maxillipede; T, 4th–8th thoracic appendages (future walking legs). (After F. Müller.) ]

Some of the Peneid larvae take on very peculiar forms, e.g. the Zoaeae of the Sergestidae, which often develop the most wonderful spines all over the body.

FIG. 109.—Mysis stage in the development of Peneus, sp. A.2, 2nd antenna; Ab.6, 6th abdominal appendage; T, telson; Th, the biramous thoracic appendages. (After Claus.) ]

The Caridea have a greatly abbreviated metamorphosis, the larva hatching out at a late Zoaea stage with all three pairs of maxillipedes fully formed and with a fully segmented abdomen. The succeeding thoracic limbs are added in order from before backwards, though the sixth pair of pleopods appear precociously as in the Peneidea. The other swimmerets do not begin to develop until the thoracic limbs are complete. Some Caridea show a yet more abbreviated metamorphosis, e.g. the fresh-water Palaemonetes varians of S. Europe, which hatches out at the Mysis stage.

We see, therefore, in the metamorphosis of the Macrura several apparently primitive features. In the first place, a free swimming Nauplius stage is preserved in certain forms, identical in all respects with the Nauplius of the Entomostraca. Secondly, the thoracic limbs when they are first developed are biramous, thus giving rise to the characteristic Mysis stage which links the Macrura on to the “Schizopoda.” Thirdly, the order of differentiation of the segments is typically from in front backwards, the only precociously developed appendage being the sixth abdominal. None of these characters are reproduced in the higher Decapoda in which there is never a free-living Nauplius, the first larval stage being the Zoaea; a number of the thoracic pereiopods, and usually all of them, are uniramous from the start; and the whole of the abdominal segments with their limbs tend to be precociously developed before the hinder thoracic segments make a distinct appearance.

=Tribe 3. Peneidea.=

The third legs are chelate except in genera in which the legs are much reduced. The third maxillipedes are seven-jointed, the second maxillipedes have normal end-joints, and the first maxillipedes are without a lobe on the base of the exopodite. The pleura of the first abdominal segment are not overlapped by those of the second. The abdomen is without a sharp bend. The branchiae are usually not phyllobranchs.

=Fam 1. Peneidae.=—The last two pairs of legs are well developed, and there is a nearly complete series of gills. Cerataspis, a pelagic form. Parapeneus, Peneus, Aristaeus, etc.

=Fam. 2. Sergestidae.=—The last or last two pairs of legs are reduced or lost. The gill-series is incomplete or wanting. Sergestes possesses gills, and the front end of the thorax is not greatly elongated. Lucifer has no gills, and the front of the thorax is greatly elongated, giving a very anomalous appearance to the animal. All the members of this family are pelagic in habit.

=Fam. 3. Stenopodidae.=—One or both legs of the third pair are longer and much stouter than those of the first two pairs. On a number of small anatomical points this family, including the littoral genus Stenopus from the Mediterranean and other warmer seas and Spongicola commensal with Hexactinellid sponges from Japan, is separated by some authors in a Tribe by itself.

=Tribe 4. Caridea.=

The third legs are not chelate. The third maxillipedes are 4–6 jointed, the end-joint of the second maxillipede nearly always lies as a strip along the end of the joint before it, and the first maxillipedes have a lobe on the base of the exopodites. The pleura of the second abdominal segment overlap those of the first. The abdomen has a sharp bend; the branchiae are phyllobranchs.

=Fam. 1. Pasiphaeidae.=—In this family the end-joint of the second maxillipedes is normally formed, and exopodites are usually present on all the thoracic limbs. Rostrum small or wanting. Rather numerous genera are known, most of which inhabit the deep sea, though a few come into the littoral zone. Pasiphaea chiefly in the deep sea, Leptochela in the tropical littoral zone.

=Fam. 2. Acanthephyridae.=—The end-joint of the second maxillipede is modified as in other Caridea, and the rostrum is very strong and serrate, but in the presence of exopodites, and in the form of the mouth-parts, this family agrees with the preceding. It is also a characteristic deep-sea family. Acanthephyra, Hymenodora, Nematocarcinus, etc.

=Fam. 3. Atyidae.=—This is an entirely fresh-water family, especially characteristic of the rivers and lakes of the tropics, some of the forms being exceedingly large and taking the place of the Crayfishes in these waters. Characteristic of this family is the fact that the fingers of the chelae are spoon-shaped, and carry peculiar tufts of bristles. Exopodites are present on the thoracic limbs of some of the genera (Troglocaris, Xiphocaris from Australia and the Malay Islands, Atyephyra from S. and W. Europe), but are absent in others. Caridina, widely spread and common in Indo-Malay and Africa; Atya from West Indies, West Africa, and Pacific Islands.

=Fam. 4. Alpheidae.=—The exopodites are absent, and the rostrum is absent or very feeble. The chelae are powerful, and usually very asymmetrically developed. Alpheus has an enormous number of species which live chiefly in the tropical seas, where they haunt especially the coral-reefs, making their homes among the coral or in sponges, etc. Although occurring in the Mediterranean they penetrate very rarely into colder seas.

=Fam. 5 Psalidopodidae.=—This family, characterised by the absence of chelae on the second thoracic limbs, which carry instead a terminal brush of hairs, and by the rudimentary condition of the eyes, is represented by the genus Psalidopus from the deep waters of the Indian Ocean.

=Fam. 6. Pandalidae.=—The first thoracic limb is without chelae, only six-jointed. The rostrum is large and toothed. The genus Pandalus has numerous representatives in the northern littoral, P. annulicornis being one of the prawns most commonly met with in the fish-markets.

=Fam. 7. Hippolytidae.=—The first and second thoracic limbs bear chelae, the carpus of the second being divided into two or more segments. The first pair of chelae are not distinctly stronger than the second. Virbius has many species in the littoral zone of all seas, and one species, V. acuminatus, is pelagic. Hippolyte also has numerous littoral forms distributed all over the world, but chiefly in the arctic or subarctic seas. H. varians, common on the English coasts, shows interesting colour-reactions to its surroundings.

FIG. 110.—Glyphocrangon spinulosa, from the right side, × 1. (From an original drawing prepared for Professor Weldon.) ]

=Fam. 8. Palaemonidae.=—The first two pairs of legs are chelate, the carpus of the second not being subdivided. Palaemon serratus, a very common prawn in the British littoral. Palaemonetes in the brackish and fresh waters of Europe and N. America.

=Fam. 9. Glyphocrangonidae.=—The first pair of legs are subchelate, the carpus of the second pair is subdivided, and the rostrum is long. Glyphocrangon (Fig. 110) with numerous species entirely confined to deep water.

=Fam. 10. Crangonidae.=—The first pair of legs are subchelate, the carpus of the second pair is not subdivided, and the rostrum is short. Crangon vulgaris is the common Shrimp of the North Sea.

=Tribe 5. Loricata.=

FIG. 111.—Dorsal view of Scyllarus arctus, × ½. (From an original figure prepared for Professor Weldon.) ]

FIG. 112.—Embryonic area of developing Palinurus quadricornis. Ab.1, 1st abdominal segment; E, compound eye; E′, median simple eye; L, upper lip; L′, lower lip; M, mandible; Mx.1, Mx.2, 1st and 2nd maxillae; Mxp.1, 1st maxillipede; T, 6th (antepenultimate) thoracic appendage. (After Claus.) ]

The Loricata include the Langouste (Palinurus) of the Mediterranean coasts, which replaces there the Lobster of the North Sea as an article of food, and the peculiarly shaped Scyllarus arctus (Fig. 111), which is also prized in the Mediterranean as a delicacy. The bright red “Crayfishes,” Panulirus and Iasus, of the Australian coasts are also largely used as food. Besides its peculiarity in shape, S. arctus has remarkable scales on the second antennae in place of flagella. The larva hatches out as the so-called Phyllosoma, which must be regarded as a greatly flattened and modified Mysis stage.

FIG. 113.—Phyllosoma larva of Palinurus, sp. × 5. Ab, Abdomen; Mxp, 3rd maxillipede; T, antepenultimate (6th) thoracic appendage. (After Claus.) ]

In the embryo of Palinurus just before hatching (Fig. 112) we can recognise the limbs of the head and thorax normally developed in order. There are present three thoracic limbs, besides the maxillipedes. When the Phyllosoma hatches out the first maxillipedes have become quite rudimentary, and the second much reduced, while the second antennae and second maxillae are also reduced in size. The metamorphosis is completed by the re-development of the limbs and segments that have been secondarily suppressed during larval life, and by the appearance of the pleopods.

This process is again met with in the Squillidae (p. 143), but it resembles the suppression, in so many Decapodan metamorphoses, of anterior limbs and the precocious development of segments and limbs lying posteriorly. In the ordinary Decapoda, however, the suppressed limbs are merely not formed till later; while in the Loricata the limbs develop in the correct order, and subsequently degenerate. It is natural to wonder whether the condition of affairs in the Loricata represents the primitive process, and whether the precocious development of segments in the other Decapoda owes its origin to these animals having once had the direct mode of development when the segments were formed in the proper order, and to their having subsequently acquired the larval stages first of all by the degeneration, and then by the suppression of certain segments which were not of use during larval life. The complete metamorphosis, however, of the Peneidea, in which the segments and limbs appear in the right order, rather goes to show that this is the primitive mode of development in the Decapoda, and that the disarrangement in the order of appearance of the segments, both in the Squillidae and in the Loricata and other Decapods, has been independently acquired in the two cases to meet the needs of the larval existence.

=Fam. 1. Palinuridae.=—The cephalothorax is subcylindrical, the eyes are not enclosed in separate orbits formed by the edge of the carapace, and the second antennae possess flagella. Palinurus, with P. elephas, the European Rock Lobster or Langouste. Iasus with two species in the Antarctic littoral; Panulirus in the tropical littoral.

=Fam. 2. Scyllaridae.=—The cephalothorax is depressed, the eyes are enclosed in separate orbits formed by the edge of the carapace, and the second antennae have flat scales in the place of flagella. Scyllarus (Fig. 111), with the European S. arctus; Ibacus in rather deep water with several species, chiefly found in the southern hemisphere.

=Tribe 6. Thalassinidea.=

This tribe is included by some authors in the Anomura, and held to be closely related to the Galatheidea, but the unreduced abdomen is carried straight and unflexed, and gives a very Macrurous appearance to the animal. The Anomurous characters are the frequent reduction or absence of the antennal scale, the fact that only the first two pairs of pereiopods are ever chelate, and the reduced series of gills. The body is symmetrical, but the first pair of chelae is always highly asymmetrical. The posterior pairs of pereiopods, although small, are not characteristically reduced as in the Anomura. The animals belonging to this Tribe attain two or three inches in length, and generally burrow in sand or mud either in the littoral zone or in deeper waters; at the same time they can swim with considerable activity by means of the pleopods.

=Fam. Callianassidae.=—Callianassa subterranea is common at Naples, Gebia littoralis in the North Sea.

=Sub-Order 2. Anomura=

In this division are included the so-called Hermit-lobsters and Hermit-crabs, in which the condition of the abdomen is roughly intermediate between that of the Macrura and that of the Brachyura. It is not much reduced in size, and the pleopods of the sixth pair are fairly well developed, but it is usually carried flexed towards the thorax, and is never a powerful locomotory organ as in the Macrura. The antennal scale, if present at all, is a mere spine, not the large leaf-like structure of the Macrura; and there is never a partition between the two first antennae as in the Brachyura.

The last or last two pairs of pereiopods are reduced, and are turned on to the dorsal surface or carried inside the branchial chamber; but this curious character is met with again in certain Brachyura (Dromiacea and Oxystomata).

=Tribe 1. Galatheidea.=

FIG. 114.—Dorsal view of Munidopsis hamata, × ½. (From an original figure prepared for Professor Weldon.) ]

These are symmetrical crabs with a long carapace; the abdomen, which is as broad as the carapace, is always carried flexed under the thorax, and the sixth pair of pleopods are expanded to form with the telson a fan-like tail. The most anterior pereiopods are always much elongated and chelate; while the last pair are much reduced, and either turned up on to the dorsal surface, or else carried in the branchial chamber. The exact meaning of this last characteristic in these forms is doubtful; some of the species are said to carry shells temporarily upon their backs, a proceeding probably assisted by the last pair of thoracic limbs, while in others their limbs may be used for cleaning out the branchial chamber. Most of the Galatheidea, for instance, the common Porcellana and Galathea, are littoral animals, and may be found hiding under stones and in crevices on the shore; but a number occur in deep water, e.g. Munida and Munidopsis.

FIG. 115.—Zoaea of Porcellana, × 20. T, Telson. (After Claus.) ]

The shallow-water species have ordinarily developed eyes; the various species of Munida, which occur in fairly deep but by no means abyssal regions, have usually very large and highly pigmented eyes; while in Munidopsis, which is characteristic of very deep water, the eyes are degenerate and colourless, as shown in Fig. 114.

The Zoaeae, or young larval stages of the Galatheidea, are characterised by the immense length of the spines upon the carapace (Fig. 115). The young Zoaea which hatches out from the egg resembles in other respects that of the Brachyura. The Metazoaea, however, differs from that of the Brachyura in the fact that the third maxillipede is first present as a biramous swimming organ, and at its first appearance is not developed in its definitive form. The other thoracic limbs are not schizopodous when they appear, and indeed in nearly all respects the development proceeds as in the Brachyura.

=Fam. 1. Aegleidae.=—The gills are trichobranchiae, and there are eight arthrobranchs. There are no limbs on the second abdominal segment of the male. The abdomen is not carried folded on to the thorax. The first two characteristics separate this family from all the other Galatheidea. Aeglea laevis, a fresh-water species from the rivers of temperate S. America, is the sole representative.

=Fam. 2. Galatheidae.=—The abdomen is not folded against the thorax. The members of this family are often littoral in habit (Galathea, Fig. 116), but often go down into great depths (Munidopsis, Fig. 114).

FIG. 116.—Dorsal view of Galathea strigosa, × ½. (From an original figure prepared for Professor Weldon.) ]

=Fam. 3. Porcellanidae.=—The abdomen is folded against the thorax, and the body has a crab-like form. These are always littoral in habit, never descending into the depths. Pachycheles in the tropics, Porcellana with numerous species in all seas, P. platycheles being a common British species.

=Tribe 2. Hippidea.=

The Mole-crabs have the habit of burrowing in sand, and their limbs are peculiarly modified into digging organs for this purpose (see Fig. 117). In other respects they are seen to be closely related to the Galatheidea by the form of the carapace, the condition of the abdomen, and the reduced last thoracic limbs.

In Albunea, which is found in the Mediterranean, the first antennae are greatly lengthened and apposed to one another, and by means of a system of interlocking hairs they form a tube down which the water is sucked for respiration. The object of this arrangement is to ensure a supply of clear water, filtered from particles of sand, when the crab is buried beneath the surface, on these occasions the tip of the antennal tube being protruded above the surface of the sand. An exactly similar tube is used by the true Crab Corystes cassivelaunus, which has similar burrowing habits, but here the tube is formed from the second antennae and not from the first, so that the tubes in the two cases afford beautiful instances of analogous or homoplastic structures between which there is no homology (see p. 189).

=Fam. 1. Albuneidae.=—The first legs are subchelate; the carapace is flattened, without expansions covering the legs. Albunea with several species in the Mediterranean, West Indies, and Indo-Pacific.

FIG. 117.—Remipes scutellatus, dorsal and ventral views, × 1. (From original drawings prepared for Professor Weldon.) ]

=Fam. 2. Hippidae.=—The first legs are simple, the carapace is subcylindrical with expansions covering the legs. Remipes (Fig. 117) and Hippa in tropical or sub-tropical seas.

=Tribe 3. Paguridea.=

The ordinary Hermit-crabs, common on the English as on every coast, are characterised by the fleshy asymmetrical abdomen from which all the hard matter has disappeared, and which is carried tucked away in an empty Gasteropod shell. The abdomen is spirally wound in accordance with the shape of the shell, and a firm attachment is effected by means of the sixth pair of pleopods, especially that of the left side, which is fashioned into the form of a hook and is curled round the columella of the shell; this attachment is so secure that in trying to pull a Hermit-crab out of its shell the body is torn apart before the hold gives way. The other pleopods are in a much reduced condition, being generally altogether absent from the right side of the abdomen, and often greatly reduced on the left side, especially in the male, though in the female they are still used for the attachment of the eggs.

The last two pereiopods are much reduced and are concealed inside the shell, which they help to carry. The great chelae are usually asymmetrically developed, that on the right side being much larger than that on the left, and often serving the purpose of shutting the entrance to the shell when the crab is withdrawn inside.

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.

FIG. 118.—Pylocheles miersii, × 1. =A=, End view of a piece of mangrove or bamboo, the opening of which is closed by the great chelae (c) of the Pagurid; =B=, the animal removed from its house. (After Alcock.) ]

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. 118), 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. 118, 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. 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.

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, 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.

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.=

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 (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 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.

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, 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.

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. 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 (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 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 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 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.

=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. 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.