PHOTOTAXIS IN THE SEA URCHIN 131 



the mass of spines all the tube feet which bore discs were cut 

 off with a pair of fine scissors. The specimens thus operated on 

 crawled away from the light repeatedly. Their rate of move- 

 ment was somewhat slower than that of normal sea urchins, but 

 their course away from the light was apparently as direct. 



EFFECT OF CUTTING THE NERVE RING 



Specimens in which the nerve ring is destroyed still show 

 the local responses to the illumination of particular parts of 

 the body. The spines bend toward the illuminated spot and 

 bristle up when a shadow is thrown upon them. If orientation 

 is effected by the direct action of light on the parts of the body 

 on which it impinges it is quite within the bounds of possi- 

 bility that a sea urchin with the nerve ring destroyed would 

 still be able to orient itself and crawl away from the light. The 

 spines and tube feet on the two sides would be stimulated equally, 

 those on the end away from the light would receive the least 

 stimulus, while those toward the light would be affected most. 

 A sea urchin with its independently responding motor elements 

 deprived of the control of a common co-ordinating nerve tract 

 is to a certain extent comparable to a ccenobic organism, such 

 as Pandorina or Vol vox, whose individual cells respond to light 

 in such a way as to cause a phototactic movement of the whole 

 colony. When we reduce the sea urchin to a system of indepen- 

 dently reacting parts it is a matter of interest to find what 

 happens to its phototaxis. 



Several dozen specimens were accordingly operated on, in 

 some of which the nerve ring was cut in several places ; in others 

 the internal organs were mainly removed, leaving intact, how- 

 ever, the radial nerves. The specimens subjected to the more 

 severe operation lived for several days and were able to move 

 their spines and tube feet about freely. They were exposed 

 at several different times to ordinary light, artificial light, and 

 direct sunlight, but not a single individual showed the least 

 trace of negative phototaxis. Even after several hours exposure 

 they remained in very nearly the same position in which they 

 were at first placed. 



In the specimens in which the nerve ring was cut great care 

 was taken to produce the least amount of injury compatible 

 with certainly cutting the nerve ring. A fine, sharp, bent scalpel 



