448 LESLIE B. AREY AND W. J. CROZIER 
close; one is tempted to compare this rather starthng exhibition 
to the effect of a thunder-storm in causing human famiUes to 
retire to their respective homes. On the other hand, the colonies 
which, in any given place, are the first to appear are likewise 
the first to retire, so in this way a separation of the different 
communities is effected, favorable to their exact observation. 
Independent study has convinced us that there is probably no 
'mixing up' of the individuals from different communities; in 
fact, we have never seen an instance in which an individual 
coming from one nest and carefully watched during the whole 
of its unmolested perambulations, failed to return to its original 
home. 
The process of return to the nest has a highly determinate 
aspect. The return course is direct and as straight as is per- 
mitted by the physical imperfections of the substratum. In 
creeping back to the nest an Onchidium may move toward the 
sun, although if removed from the rock and immediately placed 
on a glass plate it will be found negatively heliotropic. Helio- 
tropism has nothing to do with the direction of normal creeping 
(Crozier and Arey, '19 c). 
In the immediate vicinity of the entrance to the home cavity, 
the Onchidia frequently make use of a natural groove in the rock. 
In the case of small colonies, all the individuals may utilize this 
trail. The members of larger colonies, however, may simul- 
taneously approach the nest from several widely separated 
points. Arriving at the entrance to the nest, they crowd about 
it in an absurd orderly fashion, and without restlessness 'await 
their turn' to enter. Owing to the fact that the opening of the 
nest is usually quite small, several minutes may be required for 
an individual to insinuate its body into the opening. Hence 
the disappearance of the whole colony within the nest usually 
occupies some time. Once, however, they have collected about, 
or upon, the mass of Modiolus which commonly surrounds or 
even partially occludes the entrance to the nest, the Onchidia, 
because of the similarity of their coloration to that of Modiolus, 
may readily be overlooked. 
