566 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the blood circulates, and outside of which is the oxygenated water in 

 which they are bathed (Fig. 243). In the higher orders, as exemplified 

 by the crab, we find external gills like those of the oyster, and attached 

 to movable parts of the body, as the legs, exemplifying the association 

 of locomotor with respiratory activity. They are kept in motion to 

 bring the respiratory apparatus in contact with fresh portions of water. 

 In the crab the gills are inclosed within a cavity formed by a doubling 

 of the horny integument, and the stream of water is kept up through 



FIG. 241. RKSPIRATORY APPARATUS OF INSECT. (Carpenter.) 



A, first pair of legs ; B, first segment of thorax ; C, origin of wing ; D, second pair of legs ; E, third 

 pair of legs ; F, tracheae ; G, stigmata ; H, air-sacs. 



these by means of a valve in the exit-pipe worked by the jaws. The 

 constant movement causes a regular stream of water to issue from the 

 gill-chamber. 



In the fish the respiratory apparatus in all essentials corresponds 

 with that of the mollusk, the branchial element onl^y being modified and 

 multiplied in accordance with the higher grade of life. The gills are 

 formed of folds of membrane, between which are distributed the blood- 

 vessels, and which are suspended from two bony or cartilaginous arches 



