24 CORALS AND CORAL ISLANDS. 
ness. The loose stones on a sea-shore near low tide level 
often have Actiniz fixed to their under surface. A very few 
species swim or float at large in the ocean. 
Now and then an Actinia puts itself on the back of a 
crab, and thus secures rapid locomotion, but only at the will 
of the crab, which inay at times give it some hard rubs:—a 

CANCRISOCIA EXPANSA ST., ON THE BACK OF DORIPPE FACCHINO. 
kind of association styled conumensalism by Van Beneden, as 
the two in a sense live at the same table, without preying 
one upon the other. In the above example, from the China 
seas, the Actinia has mounted a Dorippe. The figure is from 
the Proceedings of the Essex Institute, where an account of it 
is published by Prof. Verrill; the specimen was collected by 
the zodlogist, Dr. W. Stimpson. As Prof: Verrill states, the 
Dorippe carries, for its protection when young, a small shell over 
its back, which it holds in this position by means of its two 
reversed pairs of hind legs. The Actinia appears to have fixed 
itself, when young, to the shell, and afterward, by its growth, 
spread over the back of the crab, taking the place of the shell. 
This case of commensalism, like most others, is not a mere 
chance association of species; for the two always go together, 
