Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/690

Rh 65G CRUSTACEA in Malta. Kumphius says, &quot; It loves to climb upon the roofs of houses. l More than fifty genera of fossil Crustacea, referable to the Decapoda-Brachyura, have been described by Milne - Edwards, Bell, Reuss, M Coy, H. Woodward, and others The oldest known crab is the Palceinackus longipes, H. Woodw., from the forest marble, Wilts (Quart. Journ. Geol. Soc., 1866, vol. xxii. PI. xxiv. p. 493). I. PODOPHTHALMIA: (1.) DECAPODA (b.) ANOMOURA. The irregular-tailed or Anomourous Crustaceans, of which the hermit-crab is a type (figs. 20 and 67), are excellent FIG. 67. Hermit-Crab removed from its shell (see ante, fig. 20). r, hardened ridge which bears against the columella of snail -shell: a a the appendages to which the eggs are attached. (Morse; examples both of arrested development and retrograde meta morphosis in the adult, resulting from disuse and consequent atrophy of particular parts or organs. We have seen among the Isopoda the females of the Bopyridcf, which live parasitic within the branchial chamber of other Crustaceans, or under their abdomen, or within the parietes of a Balanus, or actually within the body of a Porcellana. In the Rhizocephala we have seen the free- swimming pupa become attached to the soft body of the Pagurus, cast off its shell, lose all its limbs, and appear as a sausage-like, sack -shaped, or discoidal excrescence, without even a mouth, its body, filled with ova, attached by its antennae, which are modified into roots, that anchor it and at the same time bring it ready prepared nourishment from the juices of its hosts (see figs. 82 and 83). After such extreme retrogression, the depauperization of certain parts and organs observable in the Anomoura is easily to be understood and admitted. If we bring together for study a series of Anomourous Crustacea, we shall be at once able superficially to divide them into two sections, the Macrourous and the Brachyurous Anomoura. (Fritz Muller, Fur Darwin, p. 30). Tin s statement of the old Dutch naturalist seems most extraordinary, and needs further investigation. All the feet in Ranina seem adapted for digging, and an allied but much smaller form (Raninoides) from Trinidad, a truly marine species /s a most expert burrower into sand cr mud, going down tail foremost. According to Milne-Edwards, in Ranina the ordinary entrant orifice to the branchial cavity is altogether wanting, and the entrance is by a canal which opens beneath the abdomen. Such an arrangement seems rather to favour the notion of its fossorial habits. (A.) Irregular BRACHYURA. 1 . Lithodes. 2. Porcdlana (fig. 68.) 3. Dromia. 4. Dorippe. 5. Homola. (B.) Irregular MACROURA. 1 . Galathea. 2. Munida. 3. Pagun (fig. 67.) 4. Birgus. We have also burrowing forms which obviously are near to these, although not actually classed with them, viz., 1. Ranina. 2. Corystcs. Callianassa. Gebia. Axius. FIG. f&amp;gt;8. Porcellana platychtlei, Penn. sp. British. Habitat either under stones at low-water. In both these sections of the Anomoura we find the same peculiarity, namely, that the fifth pair of (thoracic) legs, and sometimes indeed the fourth and fifth hinder pairs, are not formed for walking, but are minute and rudimentary, and are placed above the level of the other legs. In Porcellana and Lithodes, in Galathea and Munida, the posterior legs are simply rudimentary. In Dorippe and Dromia, in Pagurus and Birgus, though still disproportionately small, they are modified into organs for holding on with. This rudimentary condition of the posterior thoracic feet in the An omoura at once recalls the last larval stages of nearly all the Malacostraca, 2 in which the hinder thoracic somites are not yet developed, or if so, are destitute of appendages, or have only rudimentary ones. The chelate form of the two posterior pairs of feet iu Dromia and Dorippe, in Homola and Birgus, being a vari able characteristic, and not present in all Anomoura, is doubtless developed as an individual specific modification, like the chelate penultimate feet in Brachyscelus and tho ante-penultimate pair in Phronima. 3 Some Anomoura have chelate terminations to a pair of their rudimentary feet, others have both pairs simple. 4 This is at once elucidated when we inquire into their economy. Dromia does not carry about a turbinated shell like Pagurus, but clothes itself with the skin of its victim, a &quot; sea-lemon &quot; (Doris) for example, or encourages a para sitic sponge of showy colour to grow upon its back, holding it in its place with its two hind pairs of rudimentary feet, just as the other true hermits hold their shells on over their soft-skinned bodies (Gosse). Professor Verrill has described a Dorippe (D. facchino}, which always carries an Actinia (the Cancrisocia expansa, St.) upon its back. Like most other cases of commensaiism this friendly association of the crab and sea-anemone was begun long ago and has been regularly adhered to. When young the Dorippe carries a small shell (? the half of a bivalve), which it holds in position by means of its two hind pairs of legs. The Actinia fixes itself when young to the shell, and afterwards by its growth completely conceals the carapace of the crab, replacing the shell which is, no 2 In Palcemon and Penccus (Macroura), in Dvlichia, Caprella, Ligin, and Asellus (Isopoda), in Lestrigonus (Amphipoda), and even in the Brachyura where the young apparently undergo little transformation when compared with the less cephalized forms, the thoracic somites aie developed last, and then from before backwards. (See Spence Bate on the Development of Decapod Crustacea, 1857, Phil. Trans. 1858, p. 595.) 3 In Cenibola Diogenes, one of the land-hermits, the penultimate pair of feet are furnished with curious rasp-like surfaces to their ex tremities, to enable it to hold on to the smooth inner surface of the spiral shell it has chosen for a habitation. 4 In the Ilyperice the youngest larvae cannot swim ; they are helple?s little animals which cling firmly by their chelate feet to the swimming laminoc of the Medusa in the gill-cavity of which they live when adult ; the adults lose the prehensile character of the feet and acquire the power of swimming.