106 CRUSTACEA. 



Decapoda, are mouth organs, are transformed in the Amphi- 

 poda into legs, but also that the part of the body which, in 

 the former, constitutes the head, is composed of several seg- 

 ments soldered together. 



The name Amphipoda is given to the group in allusion to 

 another leading character — namely, that of having the legs 

 of different forms, some of them, the anterior, being claw- 

 shaped, and the others simple. Considerable variation, 

 however, occurs in the form of the claws, and also in the 

 particular legs which are clawed ; and it is upon these, and 

 various other structm-al characters, that the genera have been 

 formed. In the typical genus, Gmnmarus (Latreille), the 

 four anterior legs are formed into small claws, and the upper 

 antennae offer a character which is unique in the order, that 

 of having a small articulated seta at the internal extremity 

 of the third joint. The typical species, Gammarus pulex 

 {aquaticus, Leach), is a small leaping animal, found in great 

 quantities in ditches, ponds, and springs of fresh water, 

 exceedingly active, and varying much in size. The body 

 of this animal is compressed at the sides, curved, and com- 

 posed of a series of equal-sized segments, which distinguish 

 it from the prawn and true shrimps. The anterior segments 

 are soldered together into a large thoracic shield or shell, a 

 structure which also prevails in the lobsters and crabs. Its 

 motions are effected in a manner similar to those of the spring- 

 tailed insects, Fodurce, by bringing the tail, which is termi- 

 nated by several appendages, beneath the breast, and then 

 letting it go with force, thus gi\'ing, as it were, continual 

 fillips to the water in which it resides, and by which means 

 it is impelled forwards. 



Another species {Cancer ffrossipes Lin., Corophium longicorne 

 Latreille), is much more slender in its form, and its lower 

 antennae are very strong, and as long as the body. It in- 

 habits the coasts of various Em^opean countries, and is called 

 Pernys by the inhabitants of the coast of La llochelle, where 



