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; pv 2 , 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 externally as a 

 "white body" (Fig. 3). The second or even all five pleopods may 

 sometimes also be so modified. Such isopods predominate in dry 

 situations. 



The protection of the breathing organs is most complete in myria- 

 pods and insects, with their development of a tracheal system. Inde- 

 pendently of these, the arachnids have developed the so-called fan 

 tracheae or tracheal lungs. A less perfect tracheal system has also 



