THE CLASS CRUSTACEA 323 



The body in this order is laterally compressed and usually elongate ; 

 there is no carapace ; the thoracic appendages are legs for crawling and 

 clinging ; while of the abdominal appendages the three anterior pairs are 

 feathery swimming feet, and the three posterior pairs are directed back- 

 wards and are commonly adapted for springing. 



In the typical Amphipods the four anterior pairs of thoracic legs are 

 bent forwards at the base and backwards at the tip, while in the three 

 posterior pairs these directions are reversed — a condition to which the 

 order owes it name. Most Amphipods are small. 



Fresh-water species of the widely distributed genus Gammarus (Fig. 

 136) are in some places very destructive to mosquito-larvae. 



Order Stomatopoda. 



The Locust-shrimps, or Mantis-shrimps, are a very well-characterised 

 order of predatory marine Crustacea. The anterior part of the elongate 

 body is covered by a small carapace, which, however, leaves at least the 

 4 posterior segments free ; the abdomen is long and large, and all its 

 segments except the last carry biramous swimming-plates to which the 

 gill-tufts are attached ; and the last abdominal segment is a broad 

 spinose plate and forms with the enlarged sixth pair of appendages a 

 powerful tail-fan. The first five pairs of thoracic appendages are prehen- 

 sile legs, the terminal joint of which is usually sharply serrated and 

 closes against the penultimate joint like the blade of a pocket-knife. The 

 seqond pair of these remarkable legs is greatly enlarged, and forms a 

 terrible raptorial weapon. The eyes are large and are set on movable 

 stalks, as in the Decapoda and Schizopoda. The largest Stomatopods 

 exceed a foot in length, while the smallest are about i| inches long. The 

 Stomatopods either burrow in the mud of the sea-bottom or live in 

 crevices in coral-reefs. 



