VERTEBRATA. 



13V 



The skin contains small touch organs connected with 

 afferent nerves, and these are for the recognition of pressure 

 and temperature. Possibly allied to these are the organs 

 of the lateral line, which are found only in the aquatic 

 Ichthyopsida. These organs are sometimes free on the sur- 

 face, sometimes in pits, while not infrequently the pits are 

 connected by canals running beneath the surface, with 

 openings to the exterior here and there. This line of organs 

 is plainly seen on the side of the body in most fishes. On 

 the head, however, it frequently branches greatly and be- 

 comes enormously extended in this way. The occurrence of 

 these structures in aquatic forms only would suggest that 

 their function is connected with that element; but what 

 that function is, is not well understood. 



The taste organs are within the mouth, principally on the 

 tongue. They are poorly developed in some vertebrates, 

 better in others. 



The olfactory organs are always placed in front of the 

 mouth. They consist of a membrane folded so as to expose 

 a great amount of surface, and this surface is covered with 

 the sense structure, connected with the ends of the olfactory 

 nerve. In the fishes the sacs containing this membrane 



Fro. 63. Relations of the olfactory organ, A, in fishes, B, in higher 

 vertebrates, b, brain ; i, internal nostril ; n, external nostril. The sen- 

 sory surface is folded. 



have only external nostrils, but in all others they are placed 

 at one side of a tube, which leads from the external nostril 



