NATURAL HISTORY OF ONCHIDIUM 485 
The nest was now broken into with a chisel. The three individuals 
inside were removed, and placed on a flat rock surface above the nest 
opening, and at three different points each some 46 to 50 cm. from 
the nest. All three Onchidia succeeded in effecting a return to the 
mouth of the nest. Near the nest two of them followed a slightly 
grooved trail ; this trail or channel had also been used bj^ the one return- 
ing individual described in the preceding paragraph. The channel led 
directly to the mouth of the nest. In getting into this trail, each indi- 
vidual had to change its course greatly. AVlien they had reached the 
region about the nest aperture where the rock had been broken in 
examining the interior of the cavity, the Onchidia became much 'con- 
fused' and merely wandered about on the outside rock, where they were 
left as the tide rose. 
July 2, 1914. 
A group was noted returning to a nest, and one individual was 
picked up and replaced on the rock on the opposite side of the mouth 
of its nest at a distance of 1 meter therefrom. The animal returned 
directlj' to its nest. 
In subsequent years manj^ trials of this sort were carried out, 
and always with essentially the same result. It is possible, but 
not probable, that some interesting results would have been 
obtained b}^ comparing carefully the homing capacity of Onchidia 
of different ages. In the autumn three groups of fairly distinct 
size are noticeable in the Onchidium population, so these snails 
probably live two years at least, if not more. Nevertheless, our 
experiments did not disclose any differences in homing ability 
among the individuals of different sizes. Factors which more 
noticeably affect the ability to 'home' after experimental dis- 
placement are the natural extent of the normal feeding area and 
the degree to w'hich this area is populated with nests. These 
two factors are usually correlated quite closely. Boulder-like 
rocks more or less isolated from the shore are frequently so 
eroded as to present a veritable honeycombed aspect; a rock 3 
feet by 2 in cross-section, projecting some two feet above m.l.w., 
Avas found to harbor about thirty Onchidium nests, if not more; 
less eroded rocks, often affording considerable expanses of flat 
surface, were seen to shelter an Onchidium population much 
less dense. In a habitat of the latter sort the distance limiting 
successful homing was about 1 meter, while experiments on rocks 
of the t3'-pe first mentioned (on the south side of Dyer and of 
