Ichthyopsida 451 



tains the nervous mechanisms which dominate the activities of the 

 animal. In sharks and to a small and varying extent in some "ganoids," 

 the pallium, although thin, contains nervous tissue. But in that largest 

 and most specialized group of fishes, the Teleostei, the pallium is ex- 

 tremely thin and contains no nervous elements (Fig. 324). In modern 

 lungfishes (Dipnoi) cerebral hemispheres are definitely formed, al- 

 though not strongly developed, and the thin pallium consists mainly of 

 nervous tissue. In the amphibian brain the cerebral hemispheres are 

 somewhat better developed than in Dipnoi, and the "gray substance," 

 although mostly situated in the deeper layer of the pallium (Fig. 371), 

 shows a slight tendency to extend toward its outer surface — an incipi- 

 ent cortex. In existing amphibians there are 10 pairs of cranial nerves, 

 as in fishes. There is evidence that some ancient amphibians had 12 

 pairs. 



As for sense-organs, the possession of lateral-line organs by 

 aquatic amphibians has been mentioned (p. 442). Organs of taste 

 ("taste-buds"), however, do not occur in the skin of the body as they 

 do in fishes. Although found on the skin of the head in a few amphib- 

 ians, they are ordinarily restricted to the mouth-cavity. While the 

 amphibian probably cannot "taste" food through the external skin, 

 there are present in the skin certain types of nerve-endings which are 

 stimulated by various chemical agencies such, for example, as weak 

 acids. 



The eyes are moderately well developed, except in cave-dwellers 

 and burro wers. In fishes eyelids are rarely present and even more rarely 

 movable. Some sharks have them. Amphibian eyes, when well devel- 

 oped, are usually equipped with upper and lower lids, the lower being 

 more freely movable; and there may be a third lid, the very thin 

 nictitating membrane, between the other two and the eyeball, best 

 developed in frogs and toads. 



The ear, doubtless always important as an organ of equilibration, 

 acquires in frogs and toads an important accessory apparatus which 

 increases its auditory capacity (see p. 456). In other amphibians this 

 mechanism is lacking or represented by rudiments suggesting its 

 degeneration. 



Classification 



This Class, next to Cyclostomata, is the smallest of the vertebrate 

 classes. G. K. Noble ("The Biology of the Amphibia" 1931) counts 

 "some 1900 species and 234 living genera" of frogs, toads, and sala- 

 manders. To these must be added (according to F. Nieden) 19 genera 

 and 55 species of the snakelike caecilians. The Class breaks into three 

 groups (Orders) which are widely separated and easily defined. 



