Chapter VI (1)

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=[116] 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,[117] 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[118] 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,[119] 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[120] 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.[121] 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,[122] 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.=[123]

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[124] 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,[125] 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[126] 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.[127] 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[128] 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[129] 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[130] 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_,[131] _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[132] 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.[133] 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,[134] 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.=[135]

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_,[136] 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.=[137]—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.[138]

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[139] 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.=[140]

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[141] 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.=[142]

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_[143] (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).