108 HELIOTROPISM 



nated area as 0.3 sq. mm. To accomplish this result it was necessary 

 to stimulate the anterior end of the mantle, which is more irritable, 

 photically, than other regions of the mantle. The animal contracts 

 on the non-stimulated side, and creeps away from the source of ac- 

 tivation. The whole mantle dorsum is photosensitive, local areas 

 puckering sharply into depressed pockets, secondarily extending as 

 furrows across the animal's back, when one spot is illuminated. The 

 anterior end of the foot, and the oral lappets, are more sensitive in 

 this respect. 



The tentacles, which in some Onchidia carry terminal eyes, are non- 

 reactive to hght, nor are the orienting movements of the animal inter- 

 fered with in any way by the removal of the tentacles. 



It therefore appears that although, as we have ascertained, this 

 species of Onchidium does not possess differentiated mantle eyes 

 (Semper, 1877; Stantschinsky, 1908), the physiological precursors, or 

 perhaps the remnants, of such structures are actively functional in 

 Onchidium floridanum. 



Onchidium is quite unresponsive to increase of light intensity as 

 such, but is very promptly and precisely oriented by incident 

 light having a horizontal intensity component. There is found at 

 all times and without exception a precise negative phototropism. 

 That such orientation is in no way determined by "changes of in- 

 tensity" is adequately demonstrated by the fact that these animals 

 are at the same time conspicuously reactive to shading. When the 

 light falling upon an Onchidium in air is suddenly decreased, the ten- 

 tacles are forcibly withdrawn beneath the mantle, the head is re- 

 tracted, locomotion stops, and the mantle is pressed into contact with 

 the substratum. The distribution of this mode of irritability coin- 

 cides with that evidenced in responses initiated by illumination. As 

 with Holothuria, Chiton, and some other forms, the nature of the photic 

 orientation of Onchidium is profoundly inconsistent with the differ- 

 ential sensitivity exhibited by the same animal (Crozier and Arey, 

 1918). 



2. In the Field. — Onchidium Hves during high-tide in "nests," 

 cavities in the rock containing a number of individuals, from which 

 the mollusks at low water emerge to feed upon exposed shore surfaces. 

 The individuals emanating from any one nest return simultaneously 

 to that nest before the tide rises again (Arey and Crozier, 1918). 



