NATURAL HISTORY OF ONCHIDIUM 491 
'optic' nerve in Onchidiiim seem to be as follows : The tentacular 
nerve, entering at the base of a tentacle, runs mainly to the 
periphery of the tentacle, ramifies there, and ends in intimate 
association with numerous large, clustered nerve cells near the 
tip of the tentacle; at the level of the ej^e-cup, a small ramus is 
split off from the main course of the nerve and passes to the 
eye, but an actual connection with it, such as is easily seen in 
many molluscan eyes, is exceeding difficult to demonstrate; our 
evidence seems to show, however, that a few fibers perhaps do 
actually enter the optic cup. This structural state may be 
indicative of degeneration. 
If the mantle receptors of 0. floridanum must be regarded as 
mere remnants of the original photosensitive equipment of this 
stock, the possibility of their connection with a primitive helio- 
tropic mechanism in ancestral pulmonates acquires an unprofit- 
able vagueness. We have thought it necessary to raise this 
point because it has sometimes been held that non-adaptive 
responses ''have been inherited from ancestors in which they 
were adaptive" (meaning that the mechanism for response has 
been so inherited). For Onchidium such interpretation is 
highly improbable. 
3. Neither can the heliotropism of Onchidium be dismissed as 
a mere 'laboratory product.' Some wTiters have endeavored to 
account for heliotropic orientations as found in various animals 
on the basis that determinate movements of this character must 
be the result of 'abnormal' conditions (cf. Franz, '13). It will 
be obvious that notions of this sort cannot affect the analysis of 
the mechanism of photic orientation, but can refer only to the 
role of heliotropism as an ethologic factor. It is only in a very 
limited sense that the heliotropism of Onchidium may be regarded 
as 'unnatural.' It is not that laboratory conditions artificially 
imposed determine the orientations so produced, but on the 
contrary that in surroundings other than the immediate environ- 
ment of the 'home' nest some specific factor producing central 
nervous inhibition of what may, for convenience, be termed the 
(sensory) heliotropic impulses, fails to appear. It is sufficient to 
remember that an Onchidium need only be transferred to a new 
