STAR-FISH AND SEA-URCHINS. 291 
serially from 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, while those near the tip may 
not be affected at all. Ifthe drop of acid be placed 
on the dorsal, instead of the ventral surface of the 
dise, 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, 
owing to an irritant acting on the dorsal surface, 
