80 ZOOLOGY 



of particular stimuli, and in such cases it is reasonable to infer that 

 the distinctness of the sensation cannot be very great. In 

 almost all animals, however, certain areas are specially suited 

 to be stimulated by special stimuli. 



109. Touch. There are two principal ways by which contact stimuli are 

 received among animals. Fibres of the central nervous system may pass to the 

 skin and end among its outer layers as free nerve terminations, or these fibres may 

 become intimately united with one or more of the cells of the epithelium. The 

 most common of the tactile organs in vertebrates are of the first class. Where the 

 stimulus reaches the nerve through a nervous epithelium, the epithelial cells often 

 have special developments such as hairs, bristles, and the like, whereby the 

 possibility of contact with external objects is increased (Fig. 19, A, s). The 

 appreciation of changes in temperature is also associated with the general skin 

 surface. 



no. Chemical Sense (including taste and smell). It is impossible for us to 

 distinguish between taste and smell in the lower animals. Indeed it is with 

 difficulty, in some instances, that we separate the sensations obtained from the two 

 sources even in our own case. Almost all animals seem to have some power of 



FIG. 41. 



s.c. 



FIG. 41. Statocyst in a Mollusk. n, nerve; o, statolith; s.c., sensory cells in, wall of statocyst. 



(After Claus.) 



Questions on the figure. What immediately stimulates the sensory epithelium 

 in this case? What kinds of general agencies might be supposed to produce the 

 necessary motion for this purpose? What is the present view of the function of 

 statocysts? 



appreciating the chemical condition of the medium in which they live. In aquatic 

 animals the chemical sense organs may be distributed over the surface of the 

 body. In the higher animals they collect more and more at the anterior or mouth- 

 end of the animal, with manifest advantage to the animal. In the higher land 

 forms, especially the vertebrates, the organs of the chemical sense come to lie in or 

 about the mouth and nose, the beginnings of the digestive and respiratory tracts 

 respectively. These senses are specially related to the testing of food and the 

 medium in which the animal lives. For this purpose their position in the mouth 



