86 The Animal Mind 



20. 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 re- 

 appears. Starfish were found by Romanes many years 

 ago to show, besides pronounced negative reactions to strong 

 or injurious mechanical 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 Immedi- 

 ately perceive its presence and crawl toward it. " More- 

 over, 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 and gently withdrawn as the starfish approached it, the 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 was placed on the back it remained unnoticed (642, 

 pp. 321-322). Preyer reported great individual differences 

 in the responses of starfish to food stimulation; while 

 certain specimens were unmoved by the neighborhood of 

 food, an individual of another species came from more 

 than six inches away and fell upon it (617). Whether 

 the unlikeness of behavior was due to the species 

 difference or to a difference in the degree of hunger 

 does not appear. In the holothurian Thyone briareus 

 feeding movements could not be produced by external 

 stimuli, and apparently result from the internal state 

 of hunger (565). 



