446 LESLIE B. AREY AND W. J. CROZIER 
resent the location of large mantle-glands, which give rise to a 
repugnatorial secretion; there are fourteen of these glands in 0. 
floridanum, seven on either side. 
Although at about the time of low water Onchidium is often 
abundant upon shore rocks (cf. Semper, '81; Eliot, '99), and in 
many places even conspicuous, during high tide it is invisible. 
It never wanders above high-water mark nor beneath the water 
level. In these respects it differs sharply from a much smaller 
species (Onchidiella) of intertidal habitat also found at Bermuda 
(.\i'ey and Crozier, '19 a, p. 163), the latter creeping about when 
covered by the sea, but being sheltered within dead Serpula 
tubes, barnacle shells, and the like during low tide. Eliot ('99) 
has briefly referred to the occurrence of 0. tonganum on the 
tidally exposed reefs at Apia, Samoa. 0. floridanum also lives 
upon more or less isolated rocks and islets, within Great Sound, 
but does not occur on the Bermuda 'coral' reefs, which are not 
exposed at low water. 
We were soon struck by the fact that sometimes, even when 
the tide was not high, no Onchidia were procurable in places 
known from preceding and from subsequent observations to be 
thickly populated by them. There is in fact a very exact rela- 
tion between the appearance of the Onchidia upon the rocks and 
the state of the tide. This relation involves some curious and 
precise 'homing' habits, of a kind hitherto unsuspected among 
mollusca (Arey and Crozier, '18). 
Onchidium lives in communities numbering a dozen or more 
individuals, with pocket-like 'nests' in the eroded shore rock. 
The openings to these nests are as a rule very inconspicuous. 
The mollusks creep out of their nest only when the tide has a 
little more than half ebbed, that is, about 2 to 2.5 hours before 
low tide. For any particular nest the time of emergence depends 
upon the nearness of the nest to the high-water mark. Those 
Onchidia living in nests located nearest to the high-water level 
appear in the open sooner than do those situated farther down. 
It is the actual position of the cavity of the nest itself, in rela- 
tion to the tidal level, rather than the location of its external 
opening, which regulates the moment of emergence. 
