AMPHIPODA. 87 



ORDER III. 

 AMPHIPODA. 



The Amphipoda are the only Malacostraca with sessile and 

 immovable eyes, whose mandibles, like those of the preceding 

 Crustacea, are furnished with a palpus, and the only ones 

 whose subcaudal appendages, always very apparent, by 

 their narrow and elongated form, their articulations, bifur- 

 cations, and other incisures, as well as by the hairs or cilia 

 with which they are provided, resemble false- or natatory feet. 

 In the Malacostraca of the following orders, these appendages 

 have the form of laminae or scales ; here these hairs and cilia 

 appear to constitute the branchiae. Many of them, like the 

 Stomapoda and the Laemodipoda, have vesicular bursas either 

 between their feet or at their external base, the use of which 

 is unknown. 



The first pair of feet, or that which corresponds to the se- 

 cond foot-jaws, is always annexed to a particular segment, the 

 first after the head. The antennae, which, with a single ex- 

 ception the Phronimse, are four in number, project, gra- 

 dually taper into a point, and consist, as in the preceding 

 Crustacea, of a peduncle and a single stem, or one furnished at 

 most with a little lateral branch, and usually composed of 

 several joints. The body is generally compressed and curved 

 beneath posteriorly. The terminal appendages of the tail 

 are most frequently styliform and articulated. Most of them 

 swim and leap with facility and always laterally. Some in- 

 habit springs and rivulets, and are often found in couples 

 consisting of the two sexes ; most of them however live in salt 

 water. Their colour is uniform, verging on reddish or green- 

 ish. 



They may all be comprised in a single genus, that of 



Gammarus, Fab., 



Which we may subdivide, in the first place, into three sections, 

 from the form and number of* the feet. 



