W. J. CROZIER AND L. B. AREY 109 



The natural wanderings of the Onchidia while feeding upon the 

 rocks take place without reference to the nature of the sunshine, 

 whether brilHant or dull, and bear no relation whatsoever, at any time, 

 to the direction of the incident light. The Onchidia creep out from 

 their sheltering cavities only during daylight hours, however, and 

 never at night, no matter how bright the moon. 



In the laboratory, as far as our numerous experiments have shown, 

 Onchidium moves away from a source of light under any conditions 

 of temperature (15-32°), dryness (under water, or in air), and light 

 (regardless of intensity) compatible with its active existence; where- 

 as, on its natural substratum the same individual may creep directly 

 into the horizontal rays of the setting sun, or away from them, with 

 indifference. This state of affairs is well exempUfied by the fact 

 that, if an Onchidium is picked up from its natural substratum 

 and a glass plate slipped between it and the rock, the animal orients 

 immediately away from the sun, and with machine-like precision. 

 Mere disturbance is not responsible for this phenomenon, because an 

 equivalent handhng of the moUusk, followed by its replacement on 

 the rock, does not lead to the exhibition of negative heliotropism 

 (provided the animal is restored to the rock within a certain radius of 

 its "nest"). 



Similar results are obtained if an Onchidium creeping on the shore 

 is shaded from the sun and then reilluminated from a new direction 

 by light reflected from a mirror. Momentary "hesitation" may 

 succeed such illumination, but the creature's path is not materially 

 influenced. 



3. Inhibition of Heliotropism. — We would emphasize the point that 

 the natural movements of Onchidium cannot be viewed as "contrary 

 to the dictates of the animal's heliotropism; " but that during the creep- 

 ing of the mollusk on the rock surface immediately surrounding its 

 '^home'' this heliotropism is completely inhibited. Proof of the cor- 

 rectness of this contention is seen in these facts, each of which has been 

 verified upon a number of occasions. 



(a) Removal of the oral lappets of an Onchidium, organs which 

 normally are in constant contact with the substratum, or their an- 

 esthetization by MgS04, obHterates the normal directed homing of 

 the animal; at the same time its negative heKotropism becomes a dom- 

 inant factor in the control of creeping. 



