stah-fish and sea-urchins. 291 



serially fi-om the centre of stimulation. The influ- 

 ence of the stimulus, however, diminishes perceptibly 

 with the distance from the centre. Thus, if weak 

 acid be used as the irritant, it is only the feet near 

 the bases of the rays that are retracted ; and even 

 if very strong acid be so used, it is only the feet as 

 far as one-half or two-thirds of the way up the rays 

 that are fully retracted — the remainder only having 

 their activity impaired, wdiile those near the tip may 

 not be adected at all. If the drop of acid be placed 

 on the dorsal, instead of the ventral surface of the 

 disc, the effect on the feet is found to be just the 

 converse ; that is, the stimulus here applied greatly 

 increases the activity of the feet. Further experi- 

 ments show that this effect is produced by a stimu- 

 lus applied anywhere over the dorsal aspect of the 

 animal ; so that, for instance, if a drop of acid be 

 placed on the skin at the edge of a ray, and there- 

 fore just external to the row of ambulacral feet, the 

 latter will be stimulated into increased activity ; 

 whereas, if the drop of acid had been placed a very 

 small distance past the edge of the ray, so as to 

 touch some of the feet themselves, then the whole 

 row would have been drawn in. We have here 

 rather an interesting case of antagonism, which is 

 particularly well marked in Astropecten, on account 

 of the active writhing movements which the feet 

 exhibit when stimulated by an irritant placed on 

 the dorsal surface of the animal. It may be added 

 that in this antagonism the inhibitory function is 

 the stronger ; for when the feet are in active motion, 

 owin^i' to an irritant actino- on the dorsal surface. 



