490 LESLIE B. AREY AND W. J. CROZIER 
amount of mere laboratory investigation makes it absolutely- 
certain that we may predict the movements of an animal in 
nature. In reality, however, it is largely a question of the rela- 
tive completeness with which experimentation is conducted; nor 
does it require much penetration to discover that the necessary 
degree of completeness may differ in various cases. 
A point of some interest, although perhaps unduly speculative, 
concerns the historical source of Onchidium's heliotropic machin- 
ery on the receptor side. That the possibility of heliotropic 
orientation does not entrain adaptive consequences, seems ade- 
quately demonstrated by a previous discussion (Crozier and 
Arey, '19 c). But most land pulmonates are negatively helio- 
tropic. Might it then be conceived that the sensory organs 
involved in this form of irritability are mere 'hold'overs' from the 
more ancient stock? Aside from the fact that the skin of at 
least some snails and slugs is photosensitive (Yung, '10), very 
little information useful in this connection has been discovered. 
It must be remarked that the mechanisms for sensitivity to 
Ught and to shading are seemingly closely connected, if not 
identical, in Onchidium; nothing of this sort is known for other 
pulmonates. More important, however, is the conclusion of 
Stantschinsky ('07) regarding the origin of the dorsal eyes 
(mantle-eyes) in the family of Onchidia: he has shown it prob- 
able, on general morphological grounds, that the more highly 
developed forms of mantle photoreceptors are indeed primary, 
rather than a secondary development, and that species, there- 
fore, such as 0. fioridanum, which lack the dorsal eyes have 
arrived at this condition through secondary degeneration, 
Yung ('13) holds that certain gastropods are 'bhnd,' their 
tentacular eyes being non-functional, and that this is due to the 
fact that the optic nerve fibers fail to penetrate the basement 
membrane of the retina. The lack of apparent functional 
activity in connection with the tentacle eyes of O. fioridanum 
might be of interest in relation to this conception, were it not 
for the fact that the conditions here may not involve a complete 
absence of innervation of the retinae. So far as they have been 
made out from carefully studied sections, the relations of the 
