GILLS 347 



The fringed gill appears first in the animal scale, in 

 the crayfishes, lobsters, and crabs. Here, folds of skin 



Fig. 158. — Tlie lobster, stowing fringed gills (A). 



lined with blood tubes are attached to the bases of the 

 legs and inclosed in a cavity formed by the shell. Instead 

 of a flat fold, however, the skin is broken up into tiny, 

 cylinderlike projections or fringes, thus increasing enor- 

 mously the absorbing surface without increasing the 

 space occupied by the gills. This fringing, then, is an 

 adaptation for increase of absorbent surface without in- 

 crease in space occupation, and is made necessary by the 

 increased demand for oxygen on the part of the animal. 



In these forms, the cavity is constantly supplied with 

 water, and the movement of the legs causes the gills 

 to wave about and thus bring all surfaces of the fringes in 

 contact with it. Though this gill chamber, and the appa- 

 ratus for circulating water through it, may be called ac- 

 cessory respiratory apparatus, it bears the same relation to 

 breathing as do the rib walls and diaphragm in our body. 



The fringed gill reaches its highest development in the 

 fishes. Here, the gills are placed in the sides of the head, 

 supported on arches of bone and in such a position that the 

 fish can maintain a steady stream of water over these giU 



