498 LESLIE B. AREY AND W. J. CROZIER 
individuals emanating from a given nest begin simultaneously 
to execute a direct return to the nest from which they originated. 
These animals will not enter a 'foreign' nest. 
When tested apart from their specific normal environment the 
Onchidia are always negatively phototrophic. In the natural 
state their movements are entirely independent of heliotropism. 
This independence can be obliterated by injected strychnine, 
which produces 'reversal of inhibition.' Similarly, the simul- 
taneous return to the nest on the part of the various members 
of a colony can be understood on the assumption of a 'reversal 
of inhibition' brought about by substances derived from mate- 
rials ingested while feeding. 
The impulses which, on this hypothesis, suffer central inhibi- 
tion in the outwardly creeping snail may be identified with those 
which normally control the determinate character of the home- 
ward course. These impulses probably originate in the oral 
lappets, and are taken to have the character of a 'contact odor' 
(see text) specific for each particular nest. 
This is the only hj^othesis which can account for the observed 
peculiarities of the movements of Onchidium and for the out- 
come of the experiments concerning homing reported in this 
paper. 
There is no evidence of associative or persisting memory in 
connection with homing, nor do other activities of Onchidium 
point to the existence in this form of anything approaching 
intelhgent behavior. Responses to immediate stimulations are 
adequate for the analysis of the situation. 
The negative heliotropism of Onchidium, apparently devoid 
of adaptive significance, is accounted for in terms of a photo- 
sensitive receptor system enabling these snails to respond to 
shading by an effective use of the mantle as a hold-fast, supple- 
menting the weak suctional efficiency of the foot. The existence 
of receptors making negative heliotropism possible cannot be 
understood as a condition persisting from ancestral pulmonates 
normally responding in this way. 
Mantle eyes are absent in this species, and although the ten- 
tacular eyes are perhaps of normal structure, no photic sensi- 
tivity has been discovered in connection with them. 
