Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/676

Rh 642 Podophthalmia and the Edriophthalmia we meet with numerous amphibian and terrestrial forms. No Macrouran Decapod, so far as ascertained, voluntarily quits the water, although the common lobster, the river crayfish, and the spiny lobster, all display great tenacity of life when removed from their native element. Their inability to leave the water is, no doubt, due to the fact that the carapace is less acurately fitted to the thorax than in the crabs. Certain of the Anomoura, or hermit-crabs, however, find no difficulty in adapting themselves to ter restrial conditions. The writer has kept the Cenobita Diogenes from the Antilles, tenanting an Achatina shell, alive in a Wardian case for three months, during which period he displayed great activity and most remarkable powers as a climber. These West Indian crabs are not infrequently brought over alive to England with cargoes of guano and other natural products (fig. f?0). in water, as the aquatic species of Cancer become when taken from that element and left in the air. FIG. 20. Hermit-Crab (Cenobita) in shell. (After Morse.) Dn-win refers to the abundance of hermit-crabs or* Keeling Island in the Indian Ocean (Voyage of the Beagle, p. 544), all living on the cocoa-nut, and each ensconced in some shell obtained from the neighbouring beach. On the same island is found another most remarkable and very large terrestrial Anomourous Crustacean, the Birgus latro, living in burrows at the base of the cocoa- nut trees, upon the fruit of which it subsists. This large hermit-crab seeks no artificial covering for its fleshy body, the integument of which is chiefly membranous, but has the tergal pieces of its abdominal somites calcified. It is said (by Darwin) to visit the sea nightly for the purpose of moistening its gills, and the young are hatched in the water, and pass there their earlier stages of exist ence. Many of the shore-crabs, as Grapsus, and the freshwater crab, Thelphusa, are not only able to leave the watei temporarily ; the former habitually lives out of that element, whilst many sub-tropical forms, as Gecarcinus, Gdasimus, &c., frequently live at a distance from the sea and certainly possess the power of breathing air. But ii: all land-crabs it seems essential that the gills should, if not immersed in water, at least have the air surrounding then saturated with moisture, Milne-Edwards found that Gecarcinus (fig. 21) has the membrane lining the walk of the respiratory cavity modified in a manner analogous to that observed in fishes of the order Acanthopterygiai Sometimes this provision consisted of folds and lacuna serving as reservoirs for the water ; sometimes, as in Birgus, of a spongy mass well calculated to store up th fluid necessary to keep the branchiae sufficiently moistenec to enable them to perform their functions. The swift-footed sand-crabs (Ocypoda) are exclusivelj terrestrial, and can scarcely live for a single day in water in a much shorter period, a state of complete relaxatior occurs, and all voluntary movements cease. In fact, thes hud-dwelling crabs are as truly asphyxiated by immersior FIG. 21. Gecarcinut ruricola, Land-crab of Montserrat, West Indies. Among the Amphipoda, Talitrus and Orchestia both live out of the sea, the former making a burrow for itself, tie latter choosing moist places under sea-weed, or hiding in the damp sand. With us Orchestia lives within reach of the sea spray, but in the southern hemisphere species have been met with many miles inland, choosing terrestrial plants for their abode, sometimes at an elevation of 1500 feet above the sea. Among the Isopods Sphceroma is quite a littoral form, ranging from the equator to the colder ten;perate shores. The genus Ligia also lives above high-water mark, but never far away from the sea. All the OniscidfB are terrestrial in their habits, livinguncer stones, moss, or decaying wood, and in similar damp situa tions ; they breathe air (which, however, must be saturated with moisture) by the aid of a series of respiratory branchial plates on the under side of the abdominal somites, in the same manner as in Idotea (already noticed), and in addition to this by the inspiration of air by means of certain spiracular orifices on several of the basal pairs of these same appendages (fig. 22). In a large number of the lower and simple forms, including also the parasitic Crustacea and the Cirripedia, no special organs of respha- tion exist, and we are led to conclude that this office is performed by the surface of the body and its appendages generally. MUSCULAR SYSTEM. All the muscles of the body, even those of the intestine, are composed of striated fibres. REPRODUCTIVE ORGANS. The organs of generation are easily to be discerned in most of the Crustacea, but the analogy between these parts in the male and female is so great in many genera as to need the most careful examina tion in order to discriminate between the two. Generally, however, the males may be discerned by their having cue or more pairs of limbs especially modified to assist in the marital act. With the exception of the Cirripedia the two sexes appear never to exist together in the same individual among the Crustacea. The small size and great dissimilarity of the males of some of the parasitic genera caused them to remain long unknown, and led to the error of supposing the females to be hermaphrodite, as Darwin has shown to be really (.he case in the Cirripedes. But even in this division FIG. 22. Common Woodlouse, Oitiscus asdlus.