214 



Comparative Animal Physiology 



cornis has been shown by extirpation experiments to take up through the 

 filaments oF the branchial tufts approximately 37 per cent of the total oxygen 

 consumed,'^***^ the rest entering through the integument. 



Molluscs show a great range of respiratory mechanisms with a marked 

 tendency toward the development of aerial gills. The primitive Chiton has 

 six to eight aquatic gills in each pallial groove. Bivalves generally have two 

 pairs of gills or ctenidia, each with two lamellae, situated in a water supply 

 flowing through the mantle cavity. Gastropods are gill-bearing except for the 

 pulmonate land snails, in which the "lung" is actually modified from the 

 lining of the mantle cavity. Ancula, an opisthobranch snail, with both aquatic 

 and aerial tendencies, possesses on its back a peculiar system of rigid gills 

 which can extract oxygen from both air and water. 



Crustacean gills are generally enclosed within a chamber and require water 

 circulation for adequatic respiratory exchange. In the fresh-water crab, 

 Potamonantes, for instance, the paddle-like movement of the scaphognathite 

 is indispensable, and without it respiratory exchange in the gills is inadequate 

 to support life in water.^^^ This animal can survive in relatively dry air so 

 long as the branchial chamber remains moist. 



Pearse-'^^' -"^^ has made an extensive study of the respiratory apparatus of 

 crustaceans and has correlated gill reduction with migration from marine to 

 terrestrial habitats. The hermit crab, Coenohita, can get along in air with the 

 gills removed. The ghost crab, Ocypode, has lost many gills but in their place 

 has developed branchial tufts— vascularized membranous projections of the 

 branchial cavity (Table 37). 



TABLE 37. REDUCTION OF GILL STRUCTURE IN CRUSTACEA WITH 



TRANSITION FROM MARINE TO TERRESTRIAL HABITATS 



(AFTER PEARSE^") 



*Measured by displacement of water after hardening in 1 per cent chromic acid. 



