VERTEBRATA. 249 



The development of the gills varies very much ia 

 different groups of fishes ; in some the gill slits open 

 directly to the exterior ; but in the bony fishes 

 (Teleostei) there is a common intermediate cavity, 

 covered by a structure called the operculum, or 

 gill-cover. 



The modification of the various pairs of gills is 

 believed to have given rise to very different struc- 

 tures. One of these is the Eustachian tube, which 

 in Mammalia passes from the throat to the ear, and 

 is believed to represent one of the gill-clefts. The 

 bones which support the gills are composed of paired 

 arches of bone, supported by a median rod; part of 

 this branchial skeleton persists in the higher Verte- 

 brates as the hyoid (U-shaped) bone, which supports 

 the tongue, while another part is represented by one 

 of the small bones which exist inside the ear. In 

 some fishes there are four gill-bearing clefts, and in 

 some as many as seven. 



In addition to the above modifications, it is also 

 supposed by some anatomists, that the limbs of Verte- 

 brates are derived from pairs of gills, the movable 

 filaments of which became changed into fins, and 

 travelled down the body of the fish. Others suppose 

 that the ancestors of the Vertebrates had a fin ridge 

 all along the side of the body, just as some fishes 

 have a fin ridge all along the middle of the body, 

 and that this became restricted in position, and en- 

 larged in size, forming a fin. This belief is chiefly 

 founded on the nature of the fins of Amphioxus, 



