ON CRUSTACEA. 
271 
served to run along the ground with great rapidity, choosing 
for a retreat the places most difficult of access to their pur- 
suers. Many among them, whose carapace is particularly 
tender, such as the pinnotheres, mate their habitual residence 
in the valves of certain mollusca, such as the muscles and 
pinna marina ; and others, which have a soft and vulnerable 
abdomen, place it in the cavities of deserted univalve shells, 
or in the hollows of rocks, so as to preserve it. These change 
their dwelling at certain periods as their body grows, and 
make choice of a more commodious retreat. Some macrourous 
Crustacea (the tlialassini) sink themselves into the sand or mud 
to escape from their enemies. 
The cymothoes and isopods approximating to them, the 
caligi and bopyri, which live as parasites on the bodies of the 
Crustacea, of fishes, or even under the testa of other Crustacea, 
possess an instinctive quality which causes them to distin- 
guish the beings on which they can fix, and the parts of those 
beings where they ought to place themselves in preference, 
for the purpose of finding the nutriment which is suitable to 
them. 
The land-crabs have a constant habit of uniting, at a cer- 
tain period of the year, in vast numbers, and of walking, by 
the most direct course, towards the sea, over almost every ob- 
stacle which may occur in their passage. After the deposi- 
tion of their eggs, they re-assemble and return to their former 
abode. 
Some species of different orders always live in numerous 
societies, and we may more particularly mention crangon, 
talitrus, and most of the small entomostraca, especially daph- 
nis, whose colour sometimes gives to the water a tolerably 
deep red tint. 
The crabs are courageous, and when their retreat is cut off, 
they advance their claws fiercely, and endeavour to pinch 
with their fingers, which they do very strongly, in proportion 
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