NATURAL HISTORY OF ONCHIDIUM 447 
A common habitat of Onchidium is on isolated rocks which are 
more or less completely' submerged at high water and are covered 
by a 3^ellowish-brown felt-work of algae. On such islets, and on 
more extensive protected shores of similar appearance, numbers 
of tinj^ crevices are almost invariably lined with a layer of 
]\Iodiolus,^ and these frequently contain a passageway to an 
Onchidium nest. The eroded cavity in the limestone forming 
the irregularly shaped nest is sometimes of the bigness of a man's 
head, though usually much smaller. The external openings of 
the passageways are quite inconspicuous, for they are not only 
small, but they are further masked and partially choked by the 
growth of Modiolus. It is astonishing through how small an 
opening an Onchidium can slowly make its way, as it insinuates 
itself into the tiny spaces between the mussels. An individual 
that is 5 mm. high when normally creeping can squeeze through 
the space between the edges of two glass slides held 1 mm. 
apart. In nature the process seems even more startling. WTien 
a group of Onchidia emerges from the nest the individuals 
appear one at a time in continuous series. 
Colonies were also found established at sheltered spots where 
loose stones were held together by red clay, the nest being here 
a deep crevice between two stones. This type of habitat is 
less common than that afforded by eroded limestone. 
Following the emergence from its nest, a colony of Onchidia 
wanders in various directions over the rock. Some individuals 
may creep a meter from the nest. They remain exposed for a 
certain length of time, and then, swiidtaneously , return directly 
to the nest from which they came. 
The individuals emerging from one nest sometimes become 
more or less scattered, separated, and even somewhat mingled 
with others derived from other nests. Among the components 
of any one community, however, the coincidence of the return 
to the home nest is in most cases, if not indeed in all, remarkably 
^ This Modiolus is sometimes found underneath small stones and on the under 
sides of rock slabs, \Yhere numbers of stones occur piled together. In such dark- 
ened situations the color of the mussel is not black, but, on the contrary, retains 
the reddish-brown cast of the juvenile shell. 
