ORIGIN OF THE LAND VERTEBRATES 95 



oxygenized water. The gills consist of a series of 

 pouches opening inwardly into the throat and out- 

 wardly to the exterior ; the pouches have folded 

 walls, the folds being produced into fine lamellae or 

 filaments which are charged with blood, and they are 

 supported by cartilaginous bars, or gill-bars, which 

 give rigidity to the gill walls and support the 

 branchial filaments. The number of gills varies in 

 the different kinds of fishes ; they are more numerous 

 in the Selachians (Sharks, Dogfishes and Rays), in 

 which there may be as many as seven gill-pouches, 

 while in the more modern Bony-fishes or Teleosts 

 there are only four. In the Selachians the gill- 

 pouches open separately by slits to the exterior and 

 the first gill-cleft is modified to form a special tube, 

 the spiracle, which opens in front of the first gill arch 

 or hyoid arch, immediately behind the eye. This gill- 

 cleft carries only rudimentary gills, and it serves 

 under certain conditions for the intake of water into 

 the pharynx. In the other fishes the spiracular gill- 

 cleft does not open to the exterior and is quite 

 rudimentary, while in the land Vertebrates it becomes 

 converted into the cavity of the middle ear and its 

 opening into the throat remains as the Eustachian 

 passage. This remarkable transformation will be 

 considered later. 



The gills of the fish are supplied with blood by 

 a system of afferent arteries, branching off from 



