676 



COMPARATIVE ANATOMY 



and in the same manner as the scales, and consist of a similar structure, 

 so they are assumed to be merely placoid scales modified by different 

 usage. 



All higher vertebrates inherit teeth. In birds and turtles they are 

 supposed to have been secondarily lost. 



viyc. 



B 



Fig. 401. 



Placoid scales. A, A portion of the skin of the dog fish as seen under a hand 

 lens; B, a single scale removed from the skin; C, the same in section (diagram- 

 matic), b., Base of the scale; c., the same in section; d, dentine; e., enamel; 

 p., pulp cavity. D, Part of the tail of a dogfish seen from the left side, with a 

 piece of the skin removed. LI., Tube of the lateral line ; myc., myocommata or 

 septa of connective tissue; mym., myomeres. (After Borradaile.) 



E, ctenoid; F, ganoid; and G, cycloid scales. (From the Cambridge Natural 

 History; E, F after Giinther ; G, after Parker and Haswell.) 



Ganoids (Fig. 368) develop their scales (Fig. 401) from the corium 

 alone, the epidermis playing no part. Consequently the ganoid scale is 

 all dentine. Ganoid scales are shiny, which is the very meaning of the 

 term "ganoid." They are usually rhomboidal in shape and do not pos- 

 sess a cusp. In the sturgeons (Fig. 368) the scales consolidate into 

 large bony shields called scutes. The former mailed or armoured fishes 

 merely carried this consolidation to great extremes and the plates were 

 continuous. In the sturgeons the plates are not continuous but are placed 

 in rows along the back and sides so that there are large areas unpro- 

 tected. 



It is important to note at this point that in all ganoids (Polypterus, 

 sturgeons, paddle-fishes, gar-pikes, and bow-fins, Fig. 368), the plates 

 or scutes cover the entire head. The coming together of the edges forms 



