VERTEBRATES 9 



attuned to vibrations of still higher frequency, such as cause the sensa- 

 tion of sound. The ears of the lowest fishes, however, notably the 

 cyclostomes and selachians, have not reached a development high 

 enough to be stimulated by tonal vibrations, although they may be by 

 noises sufficiently loud to produce vibrations which affect the skin or 

 lateral line; the ear of such fishes is apparently merely an organ of 

 equihbrium, the maintenance of which is probably the primitive 

 function of the vertebrate ear. 



In aquatic vertebrates taste and smell are similar senses, causing 

 similar reactions and having a similar purpose, which is the detection of 

 food. Taste buds, in many fishes, especially bottom feeders, are not 

 only present in the mouth, but are distributed over the skin, so that 

 such fishes may be conscious of the presence of food which is near any 

 part of the body, the pectoral fins and barbels being especially sensi- 

 tive. Smell is probably perceived by fishes with the nostrils alone, 

 and enables the animal to scent its food at a distance. 



The eyes of fishes and amphibians are adapted to near vision only, 

 inasmuch as they have a spherical lens and lack the ciliary muscles 

 by means of which the land animals accommodate their vision to the 

 perception of distant objects. In many fishes the whole skin is also 

 sensitive to light, independently of the eyes, the head and tail being 

 more so than the trunk. 



The change in their sensory reactions which took place in vertebrates 

 when the terrestrial forms first developed from the aquatic is well 

 illustrated in the transformation of the tadpole into the frog. The 

 tadpole has the sensory equipment of a bottom fish, and undoubtedly 

 depends largely on vibrations of the water for its means of orientation; 

 the lateral line is well developed, the ear does not appear at the surface 

 of the head, there being no tympanum, and the eye is small and sunken. 

 The frog, on the other hand, depends upon sight and hearing for its 

 knowledge of its surroundings; it has very large projecting eyes, the 

 tympanum is at the surface of the head and no lateral line is present. 

 Thus, in the terrestrial vertebrates the general or contact sense, so 

 important to fishes, falls into the background as a means of orienta- 

 tion. In frogs, toads and reptiles, generally, which are the lowest of 

 them, sight is apparently the best developed special sense, and the one 

 most depended upon for information of the surroundings; also in frogs 

 and many lizards, and undoubtedly in others too, the whole skin is 

 sensitive to light, independently of the eyes. Hearing, smell and taste 

 are all poorly represented. 



