Sensory Discrimination: the Chemical Sense 81 



food by active movements ; hence, of course, they can have no 

 specific feeding reactions. Chemical sensibility, distributed 

 over the surface of the body, has been observed in lamelli- 

 branchs, a branch of the Acephala (292). Gasteropods, in- 

 cluding snails and slugs, have, owing to their active food 

 taking, more use for a chemical sense; in marine snails it 

 seems rather definitely localized in the feelers (292). Yung 

 found in the snail Helix pomatia that smell was most acute 

 at the end of the feelers, but that the animal even when 

 deprived of its feelers could distinguish perfume. Taste 

 he found best developed near the lips, and touch sensibility 

 distributed over the body, but especially toward the end of 

 the feelers (472, 474). 



22. The Chemical Sense in Echinoderms 



In the phylum of the echinoderms, under which are classed 

 starfish and sea-urchins, the " circular symmetry" of body 

 structure characteristic of the ccelenterates reappears. Star- 

 fish were found by Romanes many years ago to show, besides 

 pronounced negative reactions to strong or injurious me- V 

 chanical stimulation, what he called a sense of smell. Its 

 manifestations depended on the physiological condition of 

 the animal ; that is, upon its degree of hunger. If kept several 

 days without food a starfish would immediately perceive its 

 presence and crawl toward it. " Moreover, if a small piece of 

 the food were held in a pair of forceps and gently withdrawn 

 as the starfish approached it, the animal could be led about the 

 floor of the tank in any direction." By cutting off various 

 parts of the rays, Romanes found that "the olfactory sense 

 was equally distributed throughout their length;" and he 

 also showed that the ventral and not the dorsal surface of the 

 body was concerned, by varnishing the latter, which left the 

 reactions unaffected, and by observing that when a bit of food 



