320 
ZOOLOGY: AREY AND CROZIER 
(usually single), commonly obstructed by the growth of small mussels (Modio- 
lus). When, during daylight hours, the tide has fallen to such an extent that 
an Onchidium 'nest' has been above water-level for about half an hour, which 
frequently happens about two hours before extreme low tide (but, for many 
nests, does not occur at all during neap tides), the Onchidia in that nest 
creep out, in succession, wander some distance over the algae-covered rocks, 
on which they feed, and then (at least an hour before their nest would again 
be covered by the rising of the tide) all return to that nest. The period of 
exposure lasts about two hours. Any one community may include individuals 
of varying lengths up to 2 cm. 
The Onchidia return each to its own particular nest, even on much-eroded 
rocks and where the nests are many and close together, and that in spite of 
the fact that the individuals emanating from different nests frequently mingle 
during their wanderings. The snails comprising anyone community begin 
the return to their nest almost simultaneously, although they may be scattered 
over an area a meter in diameter and may be situated on opposite sides of the 
nest. 
Clearly, at least two distinct problems are presented by this behavior: 
First, why do the Onchidia return to the nest, and above all, why do the 
various members of a group begin almost simultaneously to make this return? 
Secondly, what is the nature of the directive control of the return journey itself? 
For the present, we discuss only the latter problem. 
Onchidia situated at some distance from their nest pursue a fairly direct path 
toward it when they begin to return; previously, their paths may have been 
quite irregular. One or more approximately linear depressions may be ad- 
hered to in the immediate vicinity of the nest itself, but this is not necessary. 
An individual taken from one side of its nest, and placed on the opposite side, 
at a distance of a meter or slightly more, frequently, exhibits little or no hesi- 
tation in turning and moving directly toward the nest. 
The possibility of vision of the entrance to the nest, of heliotropic orientation, 
or of wind influence, severally or in combination, can be perfectly excluded 
from the role of directive agents in this matter. Neither can the wetness nor 
dryness of the substratum or of the animals' tissues be important, for the be- 
havior of the snails is the same during a dry and torrid afternoon as in a 
drenching down-pour of rain. 
In an intensively studied situation where a number of Onchidium nests were 
found close together (30 to 50 cm. apart), it was seen that an Onchidium of 
one community so placed as to creep across the tiny sunken gully followed by 
the members of a neighboring colony would sometimes after hesitation, take 
this path and follow it for some centimeters; but in only one out of ten such 
instances did it actually enter the foreign nest; always there was hesitation 
and a retracing of the path, combined in several cases with an encircling jour- 
ney about the foreign entrance. Yet, neither on the natural surfaces which they 
frequent, nor on various artificial surfaces tested does an Onchidium evince 
