368 



METAZOAN PHYLA 





the female lays her eggs in masses in a shallow hole near the water 

 and coils herself about them (Fig. 249). The larval stage is passed 

 in the egg, the larva possessing three pairs of external gills which are 

 lost when it hatches. This larva swims about in the water for a while, 

 coming to the surface for air, but at length the gill clefts close, the tail 



fin is lost, and the animal becomes terrestrial, 

 leading a burrowing life. Some types of Apoda 

 are viviparous. The whole group is to be looked 

 upon as the result of a very pronounced 

 degeneration. 



396. Food. — The food of frogs and toads is 

 composed of any living animals which they can 

 secure, particularly worms and insects. These 

 are captured by means of the protrusible tongue, 

 which can be extended considerably beyond the 

 margin of the mouth and which is covered by 

 a sticky secretion (Fig. 250). Some animals are 

 also grasped by the jaws 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 mollusks but also 

 eat fish and other amphibians and will tear pieces 

 Fig. 250.— Showing the fj.^^ ^^g bodies of dead animals in the water. 



manner in which a toad ., ,. . 



takes an insect. The They are distmctly cannibalistic. 



tongue is extruded in an ex- 397^ Color Changes in Amphibia. — The skin 



ceedingly rapid movement , , . • 1 i ■ 11 



and is inverted in the of amphibians coutams color-bcaring cells, or 

 action; the insect adheres chromatophores, which in the casc of many forms, 



to its sticky dorsal surface, i 1 1 c i_ • j j ui 



which is underneath, and particularly the trogs, are ameboid and enable 

 is drawn back into the ^^e animal to modify its color (Fig. 251). There 



mouth. {Modified from . . i r • i • i, 



Dickerson, "Frog Book.") are also cclls Containing granules ot 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. 



398. 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. 252). 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 



