1 10 HELIOTROPISM 



(b) An Onchidium removed to a strange section of shore moved 

 about in a manner largely or entirely directed by the illumination. 

 The same is true of any Onchidium which has been kept in an aquarium 

 for 24 hours and is then returned to its native scene. 



(c) In favorable instances, injection of Onchidia with strychnine 

 (0.1 to 0.2 cc. of 1 per cent strychnine hydrosulf ate) , while not 

 producing more than a moderate temporary contraction, succeeded 

 by normal creeping, leaves the snail at the mercy of its heliotropism, 

 although the animal may not have been handled at all, the injecting 

 needle being simply inserted in its back and discharged. 



These facts make it evident that in nature, as far as directed pro- 

 gression is concerned, the hehotropism of Onchidium is simply in- 

 hibited, or suffers central block, probably owing to those guiding im- 

 pulses originating in the substratum and mediated through the oral 

 lappets. Strychnine, while not necessarily "converting inhibition 

 into excitation" (Cushny, 1919; Arey and Crozier, 1919), lowers the 

 central threshold of solar activation impulses. ^ 



It is improbable, furthermore, that the diurnal rhythmicity of On- 

 chidium' s appearance from its concealed rock cavities has any relation 

 to hehotropism. The tidal rhythm of appearance is such, as we have 

 learned by long observation, that even (as in summer) though two 

 periods of low water may occur during dayhght hours, the Onchidium 

 colonies creep out to feed but once in the 24 hours. On the other hand, 

 especially in winter,^ several days may pass before a tidal period 

 occurs in good dayhght; the Onchidia then may make no appearance 

 for several days. The brightness or dullness of the day has probably, 

 as far as we can detect, no influence on emergence. 



III. 



With reference to the adaptation of habits to scene of hfe, the other 

 activities of Onchidium are no less curious than its directed creepings 

 ("homing") on the shore. It might be said that it is to the best interests 



^ "Reversals" of an analagous sort, but normally determined through feeding 

 activities, are possibly involved in "homing" movements; this matter we shall 

 treat separately. 



^ O.floridanum does not hibernate, as some other species appear to do (Hirasaka, 

 1912) in colder climates. 



