ORDER AMPHIPODA. 205 



second jaw -feet, is always annexed to a particular segment, 

 the first after the head. The antennse, whose number, with 

 but a single exception (phronima) is four, are advanced, 

 gradually become slender, terminating in a point, and are 

 composed, as in the preceding Crustacea, of a peduncle, and 

 of a single stem, or accompanied at most by a small lateral 

 branch, and most frequently pluriarticulate. The body is 

 usually compressed, and curved underneath posteriorly. The 

 appendages of the end of the tail most frequently resemble 

 small articulated stylets. These Crustacea may be comprized 

 in a single genus, that of 



Gammarus, Fah., 



Which may be divided at first, according to the form and 

 number of the feet, into three sections. 



1. Those Avhich have fourteen feet, all terminated by a 

 hook, or the same number terminating in a point. 



2. Those whose number of feet is also fourteen, but in 

 which these organs, or the last four at least, are imperfect, 

 and simply natatory. 



3. Those which have but ten apparent feet. 

 The first section will be divided into two — 



The first, Uroftera, Lair., have the head thick in gene- 

 ral, the antennae often short, and simply two in number in 

 some, and the body soft. All the feet, the fifth pair at most 

 excepted, are simple. The anterior are short or small, and the 

 tail is either accompanied at the end with lateral fins, or ter- 

 minated by appendages or points, widened and bidenticulated, 

 or forked at their posterior extremity. 



In some, as in 



Phronima, Latr. 



There are but two antennae, very short, and biarticulate. The 

 fifth pair of feet is the largest of all, and terminated in a di- 



