XIII 



PHYLUM CHORDATA 



277 



owing to the acrid secretion of their cutaneous glands, and 

 their conspicuous colours serve to warn off the Birds and other 

 animals which would otherwise devour them. A red and blue 

 Nicaraguan Frog is said to show no sign of fear of the Frog- 

 eating Birds, while the edible and more plainly coloured species 

 are in constant danger. In many Toads the skin is dry and 

 covered with warts. 



An exoskeleton is present in many Gymnophiona in the form 

 of small dermal scales, and in some Anura in the form of bony 

 plates beneath the skin of the back. In the Stegocephali a very 

 complete armour of bony scutes was present, sometimes covering 

 the whole body, sometimes confined to the ventral surface. In 

 a Urodele, OnyclwdactyliLS, and in the South African Toad, 

 Xenopus, small pointed horny claws are present on the digits. 

 With these exceptions the skin is devoid of hard parts. 



Endoskeleton.- -The vertebral column is usually divisible into 

 a cervical region, containing a single vertebra devoid of transverse 



./ ; 



B 



an 



FIG. 890. Ccecilia pachynema. A, anterior extremity from the right side ; B, posterior 

 extremity from beneath, an. anus. (After Boulenger.) 



processes ; an abdominal or thoradco-lunibar region, containing a 

 variable number of vertebrae with transverse processes and often 

 with ribs ; a sacral region, containing a single vertebra, the large 

 transverse processes, or the ribs, of which give attachment to the 

 ilia ; and a caudal region, forming the skeleton of the tail. In the 

 Gymnophiona the caudal region is very short, and there is no 

 sacrum : in the Anura the caudal region is represented by a single 

 rod-shaped bone, the urostyle. The total number of vertebrae may 

 reach 250 in Urodela and Gymnophiona : in Anura there are only 

 nine vertebrae and a urostyle. 



In the lower Urodela (Fig. 891, A and B) the centra are bi- 

 concave as in Fishes : they consist of dice-box-shaped shells of 

 bone, lined at either end by cartilage (Jvk), which is continuous 

 between adjacent vertebrae. The bony shell is developed before 

 the cartilage appears, so that the vertebrae are, in strictness, 

 membrane bones. The neural arches, on the other hand, are far 



