MORPHOLOGY OF ANIMALS 



the respiratory tract. The organs of taste are, of course, in 

 or near the mouth. Among the moUusks a sense organ which 

 is probably olfactory in function, the osphradium^ is located 

 near the gills. Among the arthropods we find notable modi- 

 fications of these organs owing to the fact that the entire 

 body-surface is there covered with an impermeable chitinous 

 coat. These sense organs are here peculiar hollow tubes or 

 cones, which are borne on the anterior portion of the body, 

 usually on the antennae and mouth parts; these hairs are 

 filled with fibrillar protoplasm which connects with sense 

 cells at the base of the hair (Fig. 25, B, C, D). 



(2) Organs of hearing and equilibration are differen- 

 tiated for the reception of vibratory or mechanical stimuli ; 

 they are very widely represented throughout the animal 

 kingdom. It is advisable to consider these two organ systems 

 together, since the two functions which they subserve are 

 united in the same general organ in the vertebrates, while 

 in lower forms it is by no means easy to distinguish between 

 the two. It has long been customary to speak of all vesicular 

 sense organs containing free solid bodies as auditory in func- 

 tion, but it is much more likely that in the lower Metazoa 

 they serve to acquaint the animal with its bodily positions, — 

 that is, that they are organs of equilibration ; they are stato- 

 cysts rather than otocysts. 



Statocysts. In many respects the simplest type of organs 

 of this class is found among certain jellyfishes. It here con- 

 sists of a short tentacle situated in a depression of the ecto- 

 derm and bearing a solid body or statolith near its free end; 

 by the movement of the tentacle the hairs or protoplasmic 

 processes of surrounding sensory cells are stimulated (Fig. 



