98 



GENERAL TRIXCirLES OE ZOOLOGY 



water constantly through them (tig. 02); in the swimming ^\■orms, on the 

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

 tube (tig. ; in most amphibians (tig. 4), on each side of the neck. More 

 rareh' the digesti\e tract functions for water-breatliing; in tjie lishes, 

 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 e.xterior on the surface of the body. The water containing oxygen in 

 solution passes out through tlie gill-slits, and bathes the gill-lea\-es which 



Fig. 63. — Anterior end of TcrchcUa ndnilosa (after Jlilne EdwardsV ph. pharynx; 

 vd, dorsal, vv, ventral, blood-vessel; hr, gills; ', tentacles. 



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

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

 being ftUed 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 ilirecllv 

 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 

 trachea? 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, si). 



