24 CORALS AND CORAL ISLANDS. 



ness. The loose stones on a sea-shore near low tide level 

 often have Actiniae 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 may at times give it some hard rubs : — a 



CANCRISOCIA EXPANSA ST., ON THE BACK OF DORIPPE FACCHINO. 



kind of association styled commensalism 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 zoologist, 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, 



