TEGUMENTARY ORGANS— SKIN STRUCTURES. 461 



two are bony. The Ctenoids are spine-rayed fishes, like 

 the perch, etc. ; the Cycloids are soft-rayed fishes, like 

 the cod ; the Ganoids are sturgeons and bony pikes or 

 gars; the P 'lac oids are sharks, skates, and rays. If we 

 make the Ctenoids and Cycloids subdivisions of teleosts or 

 ordinary typical fishes, then the classification is a good 

 one as far as it goes, and has done good service in geol- 

 ogy, for it is the scales which, together with the teeth, 

 are most apt to be preserved. 



Reptile Scales. — We have taken fish scales as the type ; 

 but scales are found also in reptiles, and even in mam- 

 mals. In reptiles they may be horny, as in snakes and 

 lizards, or bony scutes, as in Crocodilia, or a combina- 



•y--- 



FlG. 322. — Diagram showing structure and mode of formation of a rattle- 

 snake's rattle: a be, horny cuticle of last joints of the vertebra \v)\ 

 a' />' c', that of last year slipped back and caught ; a" b" c" , and 

 a" b'" c'" , are still earlier cuticles. 



tion of those two, as in the shells of tortoises and tur- 

 tles. Scutes differ from scales proper in involving the 

 dermis as well as the epidermis. 



In snakes the horny, scaly cuticle is shed every year, 

 and a new one is formed by the mucous layer. This an- 

 nual skin shedding gives rise in an interesting way to 

 the rattle of the rattlesnake. The few last joints of the 

 vertebral column {a, b, c) are enlarged and consolidated 

 into an irregular mass, and covered with a horny scale 

 thicker than elsewhere on the body (Fig. 322). With 

 the skin shedding this loosens, and is shed like the 

 rest of the skin, but, being elastic, it slips back and 

 catches on the neck of the next joint. In the next 



