xm PHYLUM CHORDATA 289 



among the foliage of the plants on which they live. The brilliant 

 and strongly contrasted' hues of the spotted Salamander and of 

 some frogs are instances of " warning colours " ; the animals are 

 inedible 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 Tree-frogs the brightness of the 

 coloration varies with changes in the intensity of the light and in the 

 surroundings. 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 Stegocephala a very 

 complete armour of bony scutes was present, sometimes covering 

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

 a Urodele, Onychodactylus, 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 

 processes ; an abdominal or thoraco-lumbar region, containing a 

 variable number of vertebrae with transverse processes and often 

 with ribs ; a sacral region, containing usually 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 vertebra and a urostyle. 



In the lower Urodela (Fig. 957, A and B) the centra are biconcave, 

 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, investing bones. The neural 

 arches, on the other hand, are far more perfectly developed than in 

 any Fish, and have well-formed zygapophyses, which articulate 

 with one another by sy no vial joints. 



The Gymnophiona also have biconcave vertebrae, but in the higher 

 Urodela (Fig. 957, C and D) and the Anura absorption of cartilage 

 takes place between adjacent centra in such a way that the convex 

 end of one fits into the concave end of the next, forming a cup-and- 

 ball joint. In the higher Urodela the convexity is on the anterior, 

 the concavity on the posterior face of each centrum (D), and the 

 vertebrae are said to be ophisthoccelous : in the Anura they are 

 usually, as in the Frog, proccelous. In the Stegocephala there is 



