CHAPTER IX 

 CRUSTACEA OF THE LAND 



THERE is every reason to believe that the 

 Arthropoda, Hke the other great groups of 

 the animal kingdom, had their origin in the sea ; but 

 they must have invaded the dry land at a very early 

 period, and most of the classes into which the group 

 is divided — the Arachnids, Myriopods, and Insects 

 — are now predominantly terrestrial in their habits. 

 The Crustacea alone have remained for the most 

 part aquatic animals, and only in a comparatively 

 few cases have they succeeded in adapting them- 

 selves completely to an air-breathing existence. As 

 already mentioned, a considerable number, both of 

 marine and of fresh-water species, are more or less 

 amphibious in their habits. Thus, the common 

 Shore Crab of our own coasts and the Grapsoid 

 Shore Crabs of warmer seas voluntarily leave the 

 water and scramble about among the rocks between, 

 and even above, tide - marks. Some Crabs, like 

 Ocypode and Gelasimtis (see Plate XV.), have gone 

 farther towards becoming land-dwellers, since their 

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