THE GENERAL ANIMAL FUNCTIONS. 75 



it is with difficulty that we separate the sensations obtained from the two 

 sources even in our own case, in some instances. Almost all animals seem 

 to have some power of 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 col- 

 lect 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. The senses thus far enumerated 

 seem among the earliest developed in the animal kingdom. 



108. Hearing. It is by no means certain that the lower animals pos- 

 sess the ability to appreciate the vibrations in matter (air, water, etc.), 

 which arouse in us the sensation of sound. There are in several groups 

 of such animals organs, the structure of which would suggest that they 

 might receive vibrations of the medium in which they live. In their sim- 

 plest condition they consist of a sac (otocyst} derived from the ectoderm 

 and lined by an epithelium containing sensory cells. From these cells 



FIG. 39. 



s.c. 



FIG. 39. Otocyst in a Mollusk. n, nerve; o, otolith; s.c., sensory cells in wall of 

 otocyst. (After Claus.) 



Questions on the figure. What immediately stimulates the sensory 

 epithelium in this case? What kinds of general agencies might be sup- 

 posed to produce the necessary motion for this purpose? What is the 

 present view of the function of otocysts? 



sensory hairs extend into the cavity (Fig. 39). The cavity contains a 

 fluid which may support one or more solid particles (otoliths). With the 

 vibration of the medium the whole would be put into vibration, but the 

 inertia of the contained fluid and otoliths would cause the latter to strike 

 against the hairs and thus serve as stimuli to the sensory cells. Late 

 researches tend to prove that these structures are organs for preserving 

 equilibrium rather than of hearing. In higher forms the ear becomes im- 

 mensely more complex, but the general conditions both of origin and 



