Gammarus. 331 



separated from the rest of the shield by an imperfect 

 hinge, and enclose below a very deep sinus. Like the 

 rest of the animal, the cephalothorax is flattened laterally. 

 It bears the eyes, the first and second antennas, the 

 mandibles, the first and second maxillae, and the maxilli- 

 peds, which belong- to the fused first segment of the 

 thorax. The mouth (PL II., m.) opens from out of the sinus 

 between the lateral plates of the cephalothoracic shield. 

 In front it is protected by a small labrum, divided into 

 an upper and lower part by an imperfect hinge, and 

 fringed with numerous setse. It is guarded at the sides 

 by the scoop-like projections of the mandibles (PI. I., fig. 

 4), and behind by the maxillipeds, the basal joints of 

 which are fused, so that they form a lower lip (PI. I., fig. 

 7). The excretory organ opens on the elongated tip of 

 the antennary cone (PL II., ant. c), on the basal joint of 

 the second antenna. The eyes (PL I., fig. 15) are com- 

 pound, sessile and reniform, the posterior margin being 

 convex. The corneal cuticula, as Leydig (11) noted, is 

 smooth and without facets. 



The thorax consists of eight segments, of which the 

 first, bearing the maxillipeds, is fused with the cephalon ; 

 the remainder are free. The seven free segments (PL II.) 

 bear each a pair of legs, which normally carry branchiae 

 on the inner side of their basal joints, though in some 

 cases these are absent from the first pair. The first four 

 bear legs directed forwards and carrying enlarged coxa] 

 plates (PL II., c.p.) which gradually increase in size from 

 the first to the fourth segment, the last being notched in 

 the upper part of its posterior edge to give play to the fifth 

 leg. These coxal plates are of moderate size compared 

 with those of some other species of Gammarus. They are 

 somewhat larger in G. locusta than in G. pidex. They 

 function as accessory respiratory organs, being covered on 



