CLASS AMPHIBIA 



375 



but the teeth are not used in mastication, the food being swallowed whole. 

 Salamanders, on the other hand, have much better developed teeth, which 

 they use in biting and tearing flesh. They not only feed on worms, 

 crustaceans, insects, and moUusks but also eat fish and other amphibians 

 and will tear pieces from the bodies of dead animals in the water. They 

 are distinctly cannibalistic. 



399. Color Changes in Amphibia. — The skin of amphibians contains 

 color-bearing cells, or chromatopliores, which in the case of many forms, 

 particularly the frogs, are ameboid and enable the animal to modify its 

 color (Fig. 265). There are also cells containing granules of guanin 

 which change these colors by refraction of light. The conditions here 

 are similar, therefore, to those existing in fish. Color changes occur as a 

 result of direct stimulation of the chromatophores by light, temperature, 

 or moisture in the environment and in response to stimuli received from 

 the nervous system. 



ABC 



Fig. 265. — Diagram to illustrate modification of color by ameboid chromatophores. 

 A, pseudopodia fully extended; B, partially extended; C, contracted. In A the color 

 present in deeper layers of the skin, represented by crosslining, is obscured, and in C it 

 prevails. 



400. Nervous System and Sense Organs. — The brain of the frog 

 includes two large olfactory lobes which are united in the median line 

 and two cerebral hemispheres which are relatively larger than those 

 possessed by any forms lower than the amphibians (Fig. 266). There 

 are also two well-developed optic lobes and a medulla. The cerebellum, 

 however, is so reduced that it can hardly be distinguished; it is a trans- 

 verse mass dorsally located at the anterior end of the medulla. The 

 spinal cord is short, corresponding to the shortness of the body. Like 

 the brain it is inclosed in two membranes — a firm outer protective dura 

 mater and a more delicate inner vascvilar pia mater. The cerebrospinal 

 system includes 10 pairs of cranial nerves and also 10 pairs of spinal 

 nerves. 



The principal sense organs of the frog are the eyes, the auditory 

 organs, and the olfactory organs. In addition to the upper and lower 

 eyelids there is a third, called the nictitating membrane, which is fused 

 with the lower one. The lens is large and nearly spherical and there 



