102 BRANCH ARTHROPODA 



sometimes called " swimming crabs " because they have the 

 last pair of thoracic legs flattened and paddle-like, adapted 

 for swimming sideways quite rapidly. They have large sharp 

 lateral spines. The strong chelipeds are adapted for cutting. 

 Each of the other thoracic appendages ends in a point with no 

 forceps. 



The little "fiddler-crab" lives in salt marshes along the Atlantic coast. 

 The male has one big and one little cheliped, which he brandishes grotesquely 

 when disturbed. 



The spider crab (Macrochei'ra) of Japan sometimes measures from 12 to 

 16 feet from tip to tip of legs, but the body is only a few inches about a 

 foot in width, making them very peculiar creatures. At a little distance 

 they look like immense sprawling spiders. 



The little oyster crab, found so often in our dish of oysters, does 

 no harm to the body of the oyster, but its life within the shell insures its 

 food being brought to it by the currents of water made by the oyster to 

 bring its own food. This is a case of commensalism 1 where there is a decided 

 advantage to one animal and none, so far as known, to the other, yet the 

 intruder does no harm. 



Order III. ArthrSs'traca comprises both marine and fresh- 

 water forms. The first thoracic segment, and sometimes the 



Fig. 75. Beach flea, Gam'marus orna'tvs. (After Smith.) 



second, is fused with the head and bears maxillipeds. The eyes 

 are usually sessile. Gammarus (Fig. 75) is a fresh-water form. 

 The Pill-bug. If one searches under old boards or logs he 

 will find a small gray or brownish fourteen-footed crustacean, 

 truly terrestrial, with depressed body and with gills on the ab- 

 dominal appendages. It is called " pill-bug " from its habit 



1 See Jordan and Kellogg's " Evolution and Animal Life," p. 370. 



