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, make 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 thalassini) 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 



13 



