108 



GENERAL PRINCIPLES OF ZOOLOGY. 



addition to which the skin functions as an accessory organ of more 

 or less importance. 



Gills. The gills are usually thin-walled areas of the skin 

 which are abundantly supplied with blood-vessels, and where richly 

 branched tuftlike projections or broad leaves have grown out, thus 

 furnishing the largest possible surface for the interchange of gases; 

 these occur in such a position as to be most exposed to fresh water ; 

 in the crayfish, for example, they are on the legs, where the motion 

 drives fresh water constantly through them (fig. 61); in the 

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

 the anterior end, projecting out of the tube (fig. 62); in most 



FIG. 62. Anterior end of Terebella nebulosa. (After Milne Edwards.) ph, pharynx ; 

 -yd, dorsal, vv, ventral, blood-vessel ; br, gills ; t, tentacles. 



amphibians (fig. 4), on each side of the neck. More rarely the 

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

 Enteropneusta, and tunicates gills have been formed in connection 

 with the pharynx, its lateral walls being pierced by the gill-slits, 

 which open to the exterior on the surface of the body. The water 

 containing oxygen in solution passes out through the gill-slits, and 

 bathes the gill-filaments, which are richly provided with blood- 

 vessels. The hind-gut also in many fishes, insects, and worms 



