THE APODOUS HOLOTHURIANS 65 



animals, and the character of the object touched is appreciated to a limited 

 degree. In some cases at least food is recognized apparently by the sense of 

 touch only. Sight is seemingly limited merely to the distinguishing between 

 light and shade, and is probably confined to those species which have special 

 light-detecting organs, but it is entirely conceivable that in other species light 

 may in some way act as a stimulus. Experiment has shown that some such 

 sense as that of taste or smell is apparently possessed by those species 

 which have the special gustatory organs or cups on the inner face of the 

 tentacles, and that this sense is located in these cups. The evidence is not 

 conclusive, however, and it is not known whether any such sense exists in other 

 Synaptids or not. The sense of hearing is apparently entirely lacking, but 

 the sense of position is good, and experiment has shown that the difference 

 between going up and going down is perfectly evident to a moving Synapta. 

 There is no doubt either that this sense is located in the so-called "otocysts" 

 (positional organs) at the base of the tentacles. There is no evidence of any- 

 thing like intelligence in the actions of Synaptas, and even their movements 

 are apparently not dependent on an intact nervous system or even an unin- 

 jured circumoral nerve-ring; for the severing of the nerve-ring in half a 

 dozen places does not, after the first shock is over, interfere with normal co- 

 ordinated movements. The habit of contracting the body so strongly as to 

 rupture the body-wall is frequent, and may possibly indicate a lack of any 

 such feeling as that which we call pain, but it is out of the question to determine 

 positively whether a Synaptid feels pain or not. Stupefying by means of 

 magnesium sulphate, chloral, etc., is, as a rule, easily accomplished to such an 

 extent as to render the animal unresponsive to any sort of stimulus. The 

 difference between a sandy (or muddy) bottom and a glass (or porcelain) sur- 

 face is rather slowly detected, but leads to definite alteration of conduct and 

 movement. Changes in the composition of the water (percentage of salts, CO 2 , 

 etc.) are readily detected and also lead to altered movements, but it is doubt- 

 ful how far these are due to nervous stimulation. 



REGENERATION. The power of regeneration in Synaptids is quite remark- 

 able in some ways, but as yet very little experimental work has been done to 

 show how great it is. One or more tentacles, even the whole circle, will be 

 reproduced if external conditions are at all favorable, but if the whole oral 

 end is cut off, there seems to be no power of reproducing it. On the other 

 hand, if the body is bisected, the anterior end appears to be able to reproduce 

 the posterior portion, although the latter cannot form a new oral end for it-- 

 self. This seems to be true, no matter where the bisection is made. Appar- 

 ently it is not essential that any considerable portion of the alimentary canal or 

 blood system should be left in the regenerating part. No experiments have 

 been carried through to show how far sense-organs or nerves . can be re- 

 produced. 



