GENERA L ORGA NO LOG Y. 



129 



crab, for example, they are on the legs, where the motion 

 drives fresh water constantly through them (Fig. 58); in 

 the swimming worms, on the back; in the tube-dwelling 

 worms, at the anterior end, projecting out of the tube; in 

 most amphibians, on each side of the head. More rarely the 

 digestive tract functions for water-breathing; in the fishes, 

 Enteropneusta, and tunicatcs gills have been formed in con- 



S5 



FIG. 58 Left second foot of a crayfish with attached gill (6r). (After Huxley.) cxm, coxo- 

 podite ; bp, basipodite ; ip, ischiopodite ; nip, meropodite ; c/>, carpopodite ; //, propo- 

 dite ; dp, dactylopodite ; cxs, bristles of the coxopodite ; e, lamina of the gill. 



nection with the fore gut, its lateral walls being pierced by 

 the gill-slits, which open to the exterior on the surface of 

 the body. Through the gill-slits, the water containing 

 oxygen in suspension passes out and bathes the gill-fila- 

 ments, here richly provided with blood-vessels. The hind- 

 gut also in many fishes, insects, and worms may become an 

 accessory organ connected with breathing, from time to 

 time being filled with fresh water. 



