ON THE LOCOMOTOR SYSTEM OF ECH1NODERMATA. 
855 
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 experiments 
show that this effect is produced by a stimulus 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 therefore 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, they may be reduced to 
immediate quiescence—he., retracted—by placing another irritant on the ventral 
surface of the disc. Similarly, if retraction has been produced by placing the irritant 
on the ventral surface of the disc, activity cannot be again induced by placing another 
drop of the irritant on the dorsal surface. 
6. Luminous stimulation.— The only other observations we have to detail under 
the present section are those relating to the influence of light. We have found 
unequivocal evidence of the Star-fish (with the exception of the Brittle-stars) and the 
Echini manifesting a strong disposition to crawl towards, and to remain in, the light. 
Thus if a large tank be completely darkened, except at one end where a narrow slit of 
light is admitted, and if a number of Star-fish and Echini be scattered over the floor 
of the tank, in a few hours the whole number, with the exception of perhaps a few per 
cent., will be found congregated in the narrow slit of light. The source we used was 
diffused daylight, which was admitted through two sheets of glass, so that the thermal 
rays might be considered practically excluded. The intensity of the light which the 
Echinoderms are able to perceive may be very feeble indeed ; for in our first experi¬ 
ments we boarded up the face of the tank with ordinary pine-wood, in order to exclude 
the light over ail parts of the tank except at - one narrow slit between two of the 
boards. On taking down the boards we found indeed the majority of the specimens 
in or near the slit of light; but we also found a number of other specimens gathering 
all the way along the glass face of the tank that was immediately behind the pine- 
boards. On repeating the experiment with blackened boards, this was never found to 
be the case; so there can be no doubt that in the first experiments the animals were 
attracted by the faint glimmer of the white boards, as illuminated by the very small 
amount of light scattered from the narrow slit through a tank all the other sides of 
which were black slate. Indeed, towards the end of the tank, where some of the 
specimens were found, so feeble must have been the intensity of this glimmer, that we 
doubt whether even human eyes could have descried it very distinctly. Owing to the 
prisms at our command not having sufficient dispersive power for the experiments, and 
