142 THE STORY OF ANIMAL LIFE 



is much modified, and 1 persists as a bone, the 

 hyoid bone supporting the tongue. 



The gills of vertebrates, arranged in successive 

 pairs along the throat, are "perforating gills"; 

 that is to say, they consist essentially of holes or 

 spaces which pass right through the wall of the 

 throat. 



If we were to seek for a general character of 

 the vertebrates, besides those mentioned above, 

 that they all possess a notochord and gills, we 

 might also find it in the character of the skin. 

 Fishes, Reptiles, Birds and Mammals, all agree 

 in this, that they have a special clothing of the 

 skin scales, feathers and fur, respectively. These 

 three kinds of structure, although so widely dif- 

 fering in appearance, are practically formed all 

 in the same way, viz., by alternate ingrowths 

 and outgrowths of the skin ; the ingrowth form- 

 ing the root of the scale, hair or feather, and the 

 outgrowth its projecting part. If these infoldings 

 and outgrowths of the skin could be straight- 

 ened out into a plane surface, the skin of a small 

 vertebrate would cover an enormous area. The 

 above list excludes the Amphibia : in this class, 

 it should be mentioned, the scales have been lost, 

 and are only found in one group. 



The scales of Fishes were at one time proposed 

 as a basis of classification : large groups being 

 characterized respectively by the possession of 

 plain rounded scales (cycloids), scales fringed at 

 the posterior end (ctenoid, or comb-like) ; placoid 

 scales, consisting of bony plates, and ganoid 

 scales, large plates covered with shiny enamel. 

 These distinctions, however, were not found use- 

 ful as a guide in classification. The diagram 

 shows the elaborate scales of the common sole. 



