486 LESLIE B. AREY AND W. J. CROZIER 
Tucker's Island) showed that homing from distances greater 
than 30 to 40 cm. was not obtainable. 
The fact that the course of an Onchidium when creeping out to 
feed may be quite 'haphazard/ zigzagging here and there, while 
the homeward course is usually as direct as the substratum 
allows, as well as the findings in experiments just cited, shows 
that it is not necessary for Onchidium to follow its own slime 
track. Limpets do adhere to their own tracks (cf. Davis, '95; 
Bethe, '98; Orton, '14, etc.); Bethe (loc. cit.) thought that 
Limpets were guided in their return journeys by a sort of chemo- 
taxis, which led them to follow their own slime trails. An 
Onchidium picked up when on its homeward journey and placed 
upon a clear glass plate in diffuse light does not tend to adhere 
to its own slime track, nor to the slime tracks of other indi- 
viduals. The same result obtains with paper or the surface of a 
brick. Nor do these snails 'favor' a w^et surface over a dry one 
(glass or filter-paper). An individual from a strange section of 
the shore put down on rock near an Onchidium nest will creep 
without hesitation across fresh trails of others. 
All the facts which we have been able to gather about the 
homing of Onchidium may be brought into relation according to 
the hypothesis which we now set forth. Complete demonstra- 
tion of the validity of this notion involves further experimenta- 
tion, the nature of which we indicate. 
The Onchidia in any one colony emerge from their nest after 
the tide has fallen so far as to have left it above water level for 
about a haK hour to an hour. They scatter over the rock sur- 
face and feed. In the unfed condition certain sensory impulses 
otherwise directing and controlling the creature's movements in 
such fashion as to cause it to return to the nest are inhibited. 
The possibility of such central inhibition is given from the 
'reversal of inhibition' with respect to phototropism seen under 
strychnine action while the snail is on the surface normal to it. 
After having fed for a certain time, substances derived from 
materials ingested while feeding pass into the juices of the 
snail's body and produce a 'reversal of inhibition' so far as the 
'homing' impulses are concerned. Reversals of behavior follow- 
