Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/691

CKUSTAOEA 657 doubt, after a time either disintegrated or abandoned when the Dorippe moults. A Sagartia, associated with a hermit- crab on our own coasts, is said entirely to dissolve away the Buccinum in which the Pagurus is lodged, and to supply its place, as in the case of Dorippe, with its expanded foot. Several specimens of a small species of Pagurus, common on the French coast, have been brought to the writer, each crab tenanted in what appeared to be the shell of a small Buccinum or Nassa ; but the whole was so completely encrusted by a sponge as to leave no part visible externally. On cutting one open vertically the spiral form of the interior cavity of the shell was very distinctly seen, but the shell itself had been entirely dis solved away by the sponge. 1 A Zoanthus has been described by Duben and Koren under tho name of Mammalifera incrustata, which is commonly found para sitic on shells that are tenanted by a species of Pagurus. In all cases the shell is destroyed after a while, not by the hermit-crab, but by some process of disintegration or absorption, the diffused basal crust of the Zoophyte forming a perfect cast of it, and affording shelter to the crab. This form occurs in Shetland and in the North of England, as well as in Norway, and is regarded by Mr Hincks as distinct from Zoanthus Couchii, Gosse (Rev. T. Hincks, Ann. and Mag. Nat. Hist. 1862, p. 304). The Adamsia palliata always selects shells tenanted by Pagurus Prideauxii, but instead of adhering to the spire the "cloak-anemone" fixes itself to the smooth inner lip of the shell, so that when the hermit is feeding, the mouth of the anemone is just below that of the crab, and ready to receive any fragment he may let fall. When the Adamsia is very young, less than half -an -inch in diameter, its outline is circular ; but as it grows older it expands laterally, form ing two lobes, which creep along the mouth of the shell, until they meet and coalesce on its outer lip. The base of the cloak-anemone is then perforated, and through this opening the hermit puts out or retracts his head and legs. 8 In all the Anomoura the abdomen is more or less modified ; for instance, in the Brachyurous type, it is not closely bent under the body, as in ordinary crabs, some Porcellance (fig. 68) carrying it extended straight out ; whilst in the long-tailed forms, like Galathea, the epimera are shorter, and the segments are less arched than in the lobster. The caudal plates are also more rudimentary. lu Pagurus (fig. 67), the abdomen is naked, only a mere trace of the shelly plates remaining. In Birgus latro the sternal portion only of the abdominal somites remains. If we turn for an instant to the Thalassinidce, a family of burrowing Macroura, we find the hard and shelly epimeral pieces of the body-segments are not properly developed, and the lobes of the tail are in like manner rudimentary ; the integument of the body is extremely thin and soft (approaching that of the hermit-crabs, which like themselves live concealed in various foreign substances living and dead, e.g., shells, sponges, Actinice, &c.). 3 There can be no doubt that both in the case of the true heruiit-crabs, and in that of the burrowing Crustacea, the FIG. C9. Callianassa subterranea. Leach, a burrowing Crustacean; coast of Devon (other species are found fossil iu the Chalk, Greensand, London Clay, <tc.). non-development of the hard calcareous covering to the abdomen (in Callianassa extending to the whole body except the legs and chelae) is due to the same cause, viz., 1 They are found tenanting shells covered by Cellepora edax and by Hijdractinia. The hermit-crabs are known to break out the spiral columella of the shell they inhabit to give themselves more room. 2 Gosse, Glimpses of Ocean Life ; see also Professor Verrill s article, American Naturalist, vol. iii. 3 We may compare the differences of their tests to that which exists between a lady s white and delicate hand, encased from infancy in a ki.l glove, and the hand of a primitive savage who uses his digits con stantly for delving in the ground for roots. In the one the covering disuse of the abdomen and caudal fin for natation, and to their constant habit of living in concealment. In the case of Birgus latro, when quite young it pro. bably conceals itself in some shell, but as it grows to such a large size and becomes so enormously fat, from feeding upon the cocoa-nut, it must abandon its early disguise and conceal itself within a burrow instead. The writer has received a small hermit-crab very like Birgus, the abdomen of which was concealed in a sponge. If then the Anomoura be the descendants of certain Crustacea in which an arrested stage of development in the young has become a persistent character in the adult, or in which organs atrophied by disuse have at last come to bo suppressed or greatly modified, the conclusion seems obvious that we are dealing not with a distinct sub-order equivalent to the Brachyura or the Macrura, but with a group com posed of various irregular forms at present placed inter mediate to, but originally belonging to both these divisions. Forms allied to Homola and Dromia Homolopsis and Dromilites occur fossil, the former in the Gault and the latter in the London Clay (in fig. 70) ; no other Anomourous forms have been met with, save some legs and chelse in tho Chalk, which have been attributed to a Pagurus. Remains of Callianassa are very abundant in tho Cretaceous and Tertiary rocks of Europe, and have been found fossil even as far off as Japan. It is always tho hands which are preserved, the body being usually too delicate for fossilization. Fig. 70. 4 1, Dromilites Lamarckii, Desm. ; London Clay, Sheppey 2. f alcen- corystes Stokesii, Gault; Folkestone. 3, Eryon arctiformis, Schl. ; Lithographic stone, Solenhofen. 4, Mecocheirus longimanus, Schl. ; Lithographic stone, Solen- hofen. 5, Cyprtdea luberculata, Sby. ; Weald, Sussex. 6, Loricula pulchella, Sby. ; L. Chalk, Sussex. I. PODOPHTHALMIA : (1.) DECAPODA (c.) MACKOURA. The common lobster and prawn are excellent examples of the Macroura. In this truly aquatic type the abdomen is no longer rudimentary, as in the crabs, but is developed into a powerful organ for leaping and swimming. The body-segments are of nearly equal growth, and being compressed at the sides, or cylindrical in form, they preser.t a well-marked contrast to the crabs, or Brachyura, in which the segments are expanded laterally. The abdomen is terminated by a broad swimming tail. The Macroura are numerically very abundant in both marine and fresh water. The lobster prefers a rocky coast, and being somewhat of a gourmand in his tastes is tempted by the fisherman t n our shores to such good purpose, that as many as 25,000 live lobsters are often delivered at Billingsgate in a day. If only as many are eaten in the whole of England as in London, this would be at the rate of 50,000 per day, or membrane is thin and soft, in the other hard and horny. One might even go further and imagine that, by continued disuse, the nails would be no longer developed ; certainly they have in civilized life become less powerful as offensive weapons, and the toe nails on the feet hava really in most persons commenced to become atrophied. 4 From Professor Owen s Palccontoloyy, p. 50, fig. 10.

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