Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 44 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY ing such protection. Crustaceans with external gills enclosed in a gill chamber by the lateral parts of the cephalothorax can survive in the air for short periods, but only under favorable conditions of humidity, such as are found at the seashore, especially at night, in the tropics. Many hermit crabs and other decapods exemplify this degree of adaptation to life out of water. Those which have gone over p
Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 44 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY ing such protection. Crustaceans with external gills enclosed in a gill chamber by the lateral parts of the cephalothorax can survive in the air for short periods, but only under favorable conditions of humidity, such as are found at the seashore, especially at night, in the tropics. Many hermit crabs and other decapods exemplify this degree of adaptation to life out of water. Those which have gone over perma- nently to terrestrial life and can thereby move far away from the water, such as hermit crabs of some genera and land crabs {Gecar- cinus, etc.), have apparatus which makes possible the moistening of the gills, and keeping them from sticking together in the air; or they have supplementary breathing organs, as in the cocoanut crab, Birgus Fig. 2.—Cross section through Birgus latro: Ic, branchial or lung cover; h, heart; g, gills; re, respiratory cavity; p, pericardium; eg, branchial blood canals leading to the heart; ai-a*, lung or shell vessels leading from the heart; rt, res- piratory tufts; pvi, pulmonary vessels leading to the heart; pv2, the same near their entrance into the pericardium. After Lang. latro, which has enlarged inner surfaces of the gill chambers, with reduced gills (Fig. 2). The terrestrial isopods (Oniscoidea) which are widespread, with a considerable number of genera and species, usually occur in damp places, where the gill apparatus on the underside of the abdomen is not in danger. Among some genera of isopods, as in Porcellio and Armadillidium, an internal breathing organ, comparable with the tracheal lung of spiders, supplements the gills. The first pair of abdominal legs, which form a cover for the delicate gills, acquire an invagination in the outer skin of their terminal branches, forming a much-subdivided breathing chamber which is visible
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