chap, ix.] EXTERNAL SKELETON. 365 



which the whole or the greater part of the body is 

 amply provided with a defensive armature. Such 

 forms are Osteoglossum and its allies, in which the 

 body is closely covered with hard scales, the " cofier- 

 fishes " (Ostracion), where the hexagonal scales fit , 

 like the pieces of a mosaic, or the globe-fishes (Diodon), 

 where the whole of the globular body is covered by 

 projecting and movable spines, which, standing out 

 on erection, must most effectually protect their pos- 

 sessor. 



The protective function of the exoskeleton of the 

 true fishes is replaced in the Cyclostomata by the 

 rich supply of mucous glands to the integument ; in 

 the hag this power is carried to so great an extreme 

 that a single example placed in more than three cubic 

 feet of water is able to shed out so much mucus that 

 the whole becomes converted into a continuous viscid 

 mass; with this power of emitting a sticking secretion we 

 may compare the " cotton-spinner," where, however, 

 no observations have yet been made as to the amount 

 of the secretion. 



The Amphibia best known to us have a soft 

 unarmed integument, but the Csecilise, among recent 

 forms, have small cycloid-like scales in their integu- 

 ment, a few Urodeles have flat bony plates, and the 

 extinct Labyrinthodonta would seem to have had a 

 plentiful supply of well-developed ventral plates. 



Among the Reptilia we have thickenings which 

 may merely form epidermic scales* as in snakes and 

 lizards, or larger bony plates (scutes), as in croco- 

 diles, or very extensive pieces, as in tortoises and 

 turtles. In the Ophidia the separate scales are held 

 together by the continuous epidermic covering to 

 which they owe their origin, and the whole is ordina- 

 rily shed in one piece ; the most remarkable modifica- 

 tion undergone by them is to be seen in the rattlesnake, 

 where the cuticular scales at the hinder end of the 



