THE SENSE ORGANS 



399 



fishes, this being the definitive condition, the grooves are converted into 

 mucus-filled canals with occasional pores opening to the surface. 



Lateral-line organs, although limited to the Ichthyopsida, are of 

 special interest to morphologists, since the ear appears to be derived 

 from one or more lateral-line organs. 



Fig. 351. — Stereogram of lateral-line organs of a fish, c, lateral-line canal; In, 

 lateralis nerve; p, pores connecting with the exterior; s, scales in skin; so, sense organs 

 of lateral line. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



OLFACTORY ORGANS 



There are two chemical senses, smell and taste, but distinction between 

 the two is difficult to draw in lower animals in which differentiated organs 

 are wanting. For smell and taste aUke, chemical substances, in order to 

 affect sense receptors, must be dissolved in water. This response to dis- 

 solved chemicals is a fundamental property of organisms, as is shown by 

 such facts as that an Amoeba will engulf a protein particle but not a piece 

 of glass, and that injurious substances swept into the gullet of a Para- 

 mecium cause a reversal of ciliary action. A similar sensitivity is shown 

 by multicellular forms. A sea anemone responds differently to pieces of 

 meat and of blotting paper, yet it has no specialized olfactory or gustatory 

 organs. 



Beginning with this fundamental property of organisms, evolutionary 

 changes in the chemical sense have followed two paths, one leading to 

 the differentiation of an olfactory epithelium and the other to the forma- 

 tion of taste-buds. Both types of sense organ occur in aquatic animals, 

 and they become still further differentiated in land animals, one respond- 

 ing to chemicals suspended in the air and the other to substances dissolved 

 in water. Of the two, smell is far more delicate. In us the two senses are 

 much confused in experience, since much that we eat is not only tasted 

 but also smelled. In general we do not taste flavors such as those of 

 onion or coffee, but smell them. 



We may infer that smells and tastes are not distinguished by coelen- 

 terates, since thev are not attracted to food at a distance. Actinians 



