GENERAL INTERNAL ORGANIZATION 



45 



capture of food, as described in the previous chapter, in contrast 

 with the "running" out of the tongue by a snake and its "licking" 

 action in the mammals. 



An exact comparison can, therefore, be made between the 

 mouth and related parts in frog and man if one understands the 

 mode of development of the human embryo. When the mouth 

 cavity of a frog is compared with that of a fish, the nostrils of the 

 fish (Fig. 25) are seen to open into blind pits, which are not con- 

 nected by internal nostrils with the mouth. There is, of course, no 

 glottis, or opening from mouth to larynx, since the fish has no lungs ; 

 but the higher fishes possess a structure called the air bladder 



Fig. 25. — Gill region and air bladder of fish. 



a.b., air bladder; g, gill slits; op, outline of operculum; st, stomach. 



which is probably homologous with the lungs of air-breathing 

 vertebrates and becomes an actual lung in lung fishes (Fig. 26). 

 Again, the Eustachian tubes found in air-breathing vertebrates are 

 seemingly absent in fishes. In this region the fish exhibits a series 

 of clefts, the gill slits, lying between what are called gill bars, or 

 skeletal rods that bear the gills; through these clefts the water 

 taken in by the mouth passes to the outside. Each one of the gill 

 slits is a lateral opening from the mouth cavity in the region 

 between head and neck. In the famihar bony fishes the external 

 gill openings are covered by a flap-like operculum (Fig. 1 C, p. 10). 

 In simpler fishes, Uke the sharks, the gill slits are visible externally 

 (Fig. 1 D, p. 10). 



At first glance these gill clefts of the fish appear to have no rela- 

 tionship to the Eustachian tubes of other vertebrates, save that 



