XXI 



E. 

 ORGANS OF SPECIAL SENSE. 



Tactile organs. Gustatory organs. Olfactory organs. Auditory 

 organs. Visual organs. Organs of unknown function. 



The fundamental structure of all the organs of special sense, 

 except the visual, is so closely alike or identical, that it is some- 

 times difficult to determine to which special sense a given 

 structure shall be assigned ; for in their simplest condition they 

 consist of an epithelial cell or cells, each having at its free 

 extremity a stiff process projecting more or less into the sur- 

 rounding medium (water or air), and at its deep extremity a 

 process continuous with the nervous system. The external 

 process may, when touched, cause an excitation of the cell, when 

 it would be tactile in function; or by being bathed by some 

 special material in solution it might give rise to the sense of 

 smell or taste : or moved by sound-waves give rise to the sense 

 of hearing. Auditory organs are often combined, or may be 

 confused with organs concerned with equilibration. There is 

 usually little doubt about a visual organ, but some luminous 

 organs have been mistaken for eyes (Euphausia). 



Tactile organs. 

 Gustatory organs. 



Olfactory organs. 



. -,., Examples of each arranged in 



Auditory organs. ^ 



T7 . , taxonomic order. 



Visual organs. 



Special sense organs of 

 doubtful function. 



F. 



INTEGUMENTARY SYSTEM AND EXOSKELETON. 



The external surface of organisms coming in closest relation 

 with external surroundings is commonly modified in the following 

 directions, viz. : For the protection of deeper parts. For 



