392 Vertebrata. 



is periodically slied entire, and replaced by a new one (ecdysis). 

 This layer is harder in some regions than in others, e.g., certain 

 spots on the fore-limb of the Prog in the breeding season. Claws 

 are absent. Rounded, saccular glands, opening to the sur- 

 face, and distributed all over the body, occur in connection with 

 the skin ; they usually secrete a slimy fluid, which keeps it moist ; 

 in some forms, there are also small mucous glands, and larger 

 poison glands, which may be so closely aggregated in some 

 regions as to cause projections ; such are the " parotids " behind 

 the head in the Toad and the Land Salamander; the secretion is 

 injurious to many animals, and thus serves as a means of defence. 

 True scales, like those of many Fish, are present in the dermis of 

 many Gymnophiona ; in other forms, large membrane bones* may 

 be present in certain regions of the skin ; or there may be calcareous 

 deposits in the dermis, as in old Toads. Like Pisces, the Urodela are 

 furnished with an unpaired fin, which extends along the back for 

 some distance, sometimes even from the head, round the tail to the 

 ventral surface, as far as the anus ; it never exhibits fin rays : it is 

 usually better developed in the males than in the females, and here 

 it is most prominent during the breeding season. Except during 

 larval life (see below) it is absent from all other Amphibia. 



The skeleton is for the most part ossified, although there are, 

 as in many Fishes, considerable tracts of cartilage, especially in the 

 skull. In the Perennibranchiata and Gymnophiona, the centra are 

 amphicoelous and the notochord is large ; in others, on the 



Fig. 318. Skeleton of a Urodele (Menopoma). 



* Scales were also present in the dermis of many extinct Amphibia (Labyrinth- 

 odonta). 



