IO4 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



Crabs differ from lobsters chiefly in being formed 

 for creeping at the bottom of the sea instead of swim- 

 ming, and in the reduction of the 

 abdomen or " tail " to a rudiment, 

 which folds into a groove under 

 the enormous thorax. They are 

 the highest and largest of living 

 Crustacea : they have been found 

 at Japan measuring twenty feet 

 between the tips of the claws. 



6. Arthrostraca ; with the tho- 

 rax reduced to six or seven seg- 

 ments owing to the fusion of one 

 or two of the anterior thoracic 

 segments with the head ; eyes 

 usually sessile. This order in- 

 eludes the wood louse or sow 



U. S. coast. . 



bug (Omsciis) found in damp 



places, the slaters (Idotea) (Fig. 61), and the sand 

 fleas (Gammarus). 



FIG. 62. Antpkithoe maculata : a sand flea. 



CLASS 2. Onycophora 



This class includes only a single genus, Peripatus, 

 the species of which are all terrestrial, living in damp 

 places, and are confined mainly to the Southern Hemi- 

 sphere. Peripatus is a cylindrical, soft-bodied animal 

 resembling a caterpillar, though the body is not seg- 

 mented. There is a plainly marked head bearing a 

 pair each of eyes, antennae, and jaws. The body is 



