572 



COMPARATIVE ANATOMY 



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 responding 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. 

 (^/oLut)!'"showfng^oronasS ^c may^ infer that smells and tastes are 

 groove (o) leading from naris not distinguished by coelenterates, since they 

 S's*' Complmtil! A^atSiTof ^^^ , ^ot attracted to food at a distance. 

 Vertebrates," after Jordan- Actinians react to food placed in the tentacles, 

 vermann.) I^^^ ^^^ ^^ food placed near the mouth. 



The chemical sense receptors are therefore probably neurosensory cells 

 located in the tentacles. 



Some flatworms search for bait placed at a distance and are therefore 

 credited with an olfactory sense. In some species, the so-called olfactory 

 organs are located upon tentacles, while in others they take the form of 

 ciliated pits near the anterior end of the body. Both tentacles and pits 

 have hair cells which are connected by nerves with the brain. Some tape- 

 worms have at the anterior end of the body a transverse ciliated groove to 

 which an olfactory function is ascribed. In nemerteans, a tubular pit, 

 the cerebral organ, which is lined by ciliated epithelium and terminates 

 near the brain, has been assumed to have a like function. 



Annelids are believed to have two sorts of chemical sense organs, 

 paired ciliated "olfactory organs" near the anterior end of the body, and 

 sense buds formed of clusters of cells, scattered in various parts of the 

 body, each cell terminating in a sensory bristle. These latter are of 

 special interest, since they resemble roughly the neuromasts of vertebrates. 

 Both cuttle fish and snails go in search of food, guided by their sense 

 of smell. In land snails the olfactory organs are located, not only on the 

 tentacles, but also over the general surface of the body. The receptors 



