128 S. .1. HOLMES 



and forth slowly like long feelers exploring the region in front 

 of the body. 



When light falls on Arbacia vertically and one-half of the 

 test is shaded the animal crawls directly into the shade. If 

 placed where sunlight falls upon it obliquely from above and if 

 either the right or the left half of the body is shaded by a board 

 as the sea urchin crawls away, it travels in the general direction 

 of the rays but deviates to a considerable extent toward the 

 shaded side. If the side toward the sun is shaded and the sun- 

 light allowed to fall upon the other half, the sea urchin will 

 generally go against the rays of the sun toward the shade. The 

 sea urchin, as we might expect, tends to go away from whatever 

 side is the more intensely stimulated. 



Occasional individuals show a positive phototaxis under the 

 same conditions in which others are strongly negative. One 

 such individual made six successive straight trips toward the 

 light after which it became indifferent or slightly negative; 

 several others showed a positive reaction which was nearly as 

 persistent. In the positive reactions the spines are bent slightly 

 downward on the side toward the light and are raised on the 

 opposite side; and several of the tube feet from near the equa- 

 torial region of the test become greatly extended in the direction 

 of locomotion. In all of the positive specimens examined the 

 rate of movement was slow compared with the negative ones 

 and occurred only in rather dim light. It cannot be said, how- 

 ever, that the majority of specimens observed were positive 

 even in very dim light, although strong light produced a 

 negative reaction in all specimens in which a well marked pho- 

 totaxis occurred. 



Removal of the organs in the ocular plates which have falsely 

 been regarded as eyes produced, contrary to the statement of 

 Romanes ' for Echinus, not the least change in the reaction to light. 



REACTIONS TO SHADOWS 



Arbacia reacts to shadows in much the same way as several 

 other species of sea urchins. Pass a dark object over it and 

 the spines are erected so as to stand out at right angles to the 

 surface of the body. This reaction may be repeated three or 

 four times, after which the animal fails to respond. After 



1 Jellyfish, Starfish and Sea Urchins, p. -Til. 



