93 



GENERAL PRINCIPLES OF ZOOLOGY 



water constantly through them (fig. 62); in the swimming worms, on the 

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

 tube (fig. 63) ; in most 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-leaves which 





FIG. 63. Anterior end of Terebella nebidosa (after Milne Edwards), ph, pharynx; 

 vd, dorsal, irv, ventral, blood-vessel; br, gills; t, tentacles. 



are richly provided with blood-vessels. The hind-gut also in many 

 fishes, insects, and worms may become an accessory respiratory organ, 

 being filled from time to time with fresh water. 



Aerial Respiration. In the air-breathing animals the respiratory 

 apparatus is derived either from the digestive canal or from the skin. 

 With the vertebrates the former is the case, since the lungs, either directly 

 or by way of the trachea and bronchi, are in connection with the lumen of 

 the digestive tract. On the contrary, in snails and spiders when the term 

 'lung' is used, it refers always to an invagination or sac of the skin; the 

 tracheae of insects are similar tubes containing air, beginning at the surface 

 of the body with a hole, the spiracle or stigma, and branching internally 

 (fig. 60, sf). 



