298 ZOOLOGY sect. 



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. 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, Onychodactyht,s, and in the South African Toad, 

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

 With these exceptions the skin is devoid of hard parts. 



Sndoskeleton. — The verlehrcd column is usually divisible into 

 a cervical region, containing a single vertebra devoid of transverse 

 processes ; an abdominal or thoraco-htinbar 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. 946, A and £) 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 vertebrje 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 synovial 

 joints. 



The Gymnophiona also have biconcave vertebrae, but in the higher 

 Urodela (Fig. 946, 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 (Z>), and the vertebrae are said to be ophisthoccBlous : 

 in the Anura they are usually, as in the Frog, procoelous. 

 In the Stegocephala there is great diversity in the structure 

 of the vertebral column.' There may be well - developed 

 dice-box-shaped centra, or the neural arches may be simply 

 perched upon a persistent notochord surrounded by incomplete 



