144 ELEMENTS OF COMPARATIVE ZOOLOGY. 



spiracular cleft; in the higher vertebrates it enters into the 

 structure of the ear, giving rise to the cavity of the drum 

 and to the Eustachian tube. 



In the sharks each cleft opens separately to the exterior; 

 but in ganoids and teleosts the hyoid septum gives rise to a 

 fold (operculum) or " gill-cover," which grows back over 

 the external openings, so that there is apparently but a 

 single slit externally. A little con- 

 sideration will show that there is little 

 real modification. In the anurous 

 batrachia a similar fold is found, but 

 this unites again with the body-wall 

 behind the gills, thus enclosing the 

 external openings in an atrium, with 

 but a single opening to the exterior 

 (p. 50) . In the sauropsida and mam- 

 Fio.68. Human embryo mals gill pouches are formed in the 



(after Hertwig), with r ... 



the floor of mouth and embryo, but according to recent ob- 



and throat removed, ,, .. , ,, . 



to show the rudimen- servers these never break through, so 



tary gill-slits, g. I, r-n 



lung; n, nostril, still that no real clefts are formed. With 



connected with the ,. .. , .. . , ., 



mouth. growth all but the first pair of these 



pouches disappear, the first persisting as the Eustachian 

 tube. 



In all vertebrates above fishes, gills are supplemented 

 (batrachia) or replaced by lungs. These are paired sacs 

 richly supplied with blood-vessels, and connected with the 

 external world by means of a tube (windpipe or trachea) 

 which opens by the glottis upon the floor of the pharynx. 

 The trachea is usually strengthened by the development of 

 cartilages in its wall, some of which may become large, as 

 in the case of the human "Adam's apple." The lungs 

 themselves may be simple sacs, but usually they become 

 greatly folded, thus increasing the respiratory surface. In 



