362 THE ZOOLUGIS'. 
little or no attachment to the surface of the exoskeleton of the 
animal. 
The generic peculiarity already alluded to—namely, its slug- 
gish habits—are well illustrated in this species; and it is partly 
owing to the sedentary habits of the female in particular that the 
spores, &c., of marine animals are enabled to become planted 
in this villous covering. Mr. Carrington’s assistant, Edward 
Matthews, recently made some observations upon some specimens 
of this species which were living in one of the tanks at the 
Royal Aquarium, Westminster, and he noticed that one remained 
perfectly motionless between two little ledges of rock for two or 
three days. Thinking it was dead, he was about to remove it, 
but, finding it to be perfectly lively and well, he returned it 
to the water, whereupon it took up its former position, in which 
it was seen altogether for upwards of a week. This is a striking 
instance of the stationary habits of some of the Crustacea, and 
it is therefore not. so much a matter of surprise that they 
become, in the way they do, so assimilated in appearance to their 
surroundings. 
Many of the specimens of this species that have come before 
our notice have been so completely buried in masses of sponge 
and Alcyoniwm that it seems more than probable that their 
protective covering ultimately destroys them. ‘Thus that which 
preserves their life by protecting them from the attacks of natural 
enemies, who evidently do not recognise them under their spongy 
covering, eventually causes their death. In one specimen which 
we saw, only one leg and the abdominal segments were left free, 
and the eyes were just able to move in a deep hole in the sponge, 
which proved to be of the genus Oplilitaspongia. 
In course of time, no doubt, Pisa Gibbsii thus becomes the 
nucleus of a mass of silicious spicule which envelop and preserve 
the form of the crab. Were there any representatives of this 
peculiar type existing in the cretaceous seas, doubtless we should 
meet with their silicified tests in the flints of that formation. We 
are not aware of the existence of any allied form having hitherto 
been found in any rocks. We are, however, enabled to form some 
sort of idea as to the manner in which certain bodies may have 
become enveloped in masses of silicious spicule, and ultimately 
to have become the nucleus of a flint nodule. 
