VERTEBRATES THAT BREATHE IN WATER 



387 



essential organs of hearing contained in the capsule. This drum 

 is the equivalent of the spiracular cleft of a Dog- Fish, and it is 

 therefore clear that the backboned animals which live on land 

 have pressed an old gill-cleft into the service of their hearing 

 organs, which is in itself sufficient proof that these animals have 

 descended from aquatic gill-possessing ancestors. 



In some fishes of the Shark kind the internal openings of the 

 gill-pouches are guarded by slender projections ("gill-rakers"), 

 the use of which is to prevent foreign bodies or bits of food from 

 passing into the pouches and choking them up. 



SEA-CATS (HOLOCEPHALI) 



The members of this small group are closely related to the 

 Sharks, but are in advance of them so far as breathing organs are 

 concerned. The best-known of them is the Sea-Cat, or King of 

 the Herrings {ChimcBra monstrosd). There is here no spiracle, 



Fig. 517. — The Sea-Cat {Ckijna>ra juonsirosd), male 



and the gill-clefts, which are close together, are reduced to four 

 in number. Externally they are protected by a flap or gill-cover 

 {opei^culunt), supported by gristle (fig. 517). We shall see in the 

 sequel that various arrangements have been evolved in different 

 sorts of aquatic animals for the protection of the gills, for these 

 structures are necessarily of delicate texture (or exchange of 

 gases could not take place through their walls), and are not 

 only liable to be injured by grains of sand and the like, but are 

 also much favoured by parasites, which find in them a sheltered 

 home provided with a very rich supply of blood. 



