GAMMAEUS. 333 



(PI. II., tel.). These segments decrease rapidly in size 

 from before backwards, and their terga overlap to a great 

 extent, allowing the abdomen to be bent right forward 

 under the thorax. The terga of the first three abdominal 

 segments or metasonie are produced into large epimeral 

 plates (PI. I., fig. 13 and PI. II., epim. p.2), which increase 

 in size from before backwards and are pointed at the 

 infero-posteal corners, though the last pair are less drawn 

 out in G. pulex than in other species of Gammarus (PI. I., 

 fig. 13, epim. p. 2). In G. locusta, the last pair of 

 epimeral plates of the metasonie are considerably pro- 

 duced at the lateral corners, and bear sette along the 

 posterior edge. Each segment of the metasome bears a 

 pair of pleopods or swimming feet, consisting of a single, 

 stout, elongated protopodite, bearing two similar, many- 

 jointed rami (endopodite and exopodite) furnished with 

 long, plumose setee (PL I., fig. 12). The last three body 

 segments or urosome usually move as one, slipping 

 between the last pair of epimeral plates when flexed, and 

 forming a powerful propelling organ. They bear the uro- 

 pods (PI. II., uro. 1, 2, 3), the last pair being the largest 

 (PI. II., uro. 3), and have no epimeral plates. In G. pulex, 

 the posterior edge of each tergum bears two small dorsal 

 spines and a single lateral one; in G. locusta, each bears a 

 small dorsal projection armed with from three to five 

 spines, and having from three to four spinules on each 

 side. The telson (PI. I., fig. 11, and PI. II., tel.) is very 

 small and consists of a basal joint with two rami, each 

 bearing two apical spines and one lateral one in G. pulex; 

 in G. locusta each ramus bears three apical spines, and 

 two groups of lateral ones, the proximal group consisting 

 of three, the more distal group of two spines. The anus 

 opens below the telson and above the last pair of uropods, 

 not below the latter, as Sars (19) figures for G. neglectus. 



