SENSE ORGANS 159 



373. The sensations yielded by the olfactory organ are much 

 more various than those of the sense of taste, but at the present 

 no satisfactory analysis of olfactory sensations is possible. 

 So far as can be seen under the microscope, however, the sense 

 organs are very simple and all of the same kind. They present 

 very much the same appearance as the sensory cells of the epi- 

 dermis of nereis. The olfactory organ forms a small part of 

 the mucous epithelium, lining the nasal cavity at its upper 

 angle. The epithelium here consists of columnar cells of two 

 kinds. The first are the sensory cells which are very slender 

 and end in a group of six to eight short bristle-like tips. Below 

 the nucleus the cell narrows to a very slender nerve fibre which 

 goes to the brain. Between the sensory cells are the somewhat 

 stouter ''supporting" cells. The superficial ends of these are 

 quite regularly prismatic in form, but below the nucleus they 

 are very irregular in form. A third type of cells, called " basal 

 cells," form the deeper layer of the epithelium. 



THE ORGANS OF SIGHT 



374. Mention has already been made of the fact that sensi- 

 tiveness to light is exhibited by protozoa, which have no sense 

 organs. There are some protozoa, however, which have an 

 "eyespot," a small speck of red pigment embedded in the pro- 

 toplasm. These eyespots are found to be especially sensitive 

 to light, and must, therefore, be regarded as an exceedingly 

 simple type of light sense organs. 



375. Hydra is sensitive to light, but has no organs specially for 

 light perceptions. In some other Ccelenterates, as some of 

 the free swimming medusae, true light-sense organs are found. 



376. Among worms, again, sensitiveness to light does not 

 always indicate the presence of well-defined sense organs 

 especially constructed for this function. The earthworm is 

 more or less sensitive to light over the entire body surface, but 



