496 CRUSTACEA MALACOSTRACA. 



segments. Like the branchiae, they may, in the Laemodipoda, be 

 present on a segment which bears no other trace of an appendage. 



Of the six abdominal segments of the Amphipoda Genuina 

 the three anterior are large and bear the swimming feet, which 

 consist of two multi- articulate and setose rami borne on a stout 

 basal segment ; the three posterior appendages are directed 

 backward, and in the Gammarina form, with the terminal ab- 

 dominal segments, the powerful organ with which, by the sudden 

 extension of the abdomen, the characteristic leaps of these 

 " sand-hoppers " are effected. The telson may be simple or 

 {Gammarus) completely cleft. 



In the Laemodipoda the abdomen is reduced to a short stump, 

 with vestigial appendages (Fig. 308). 



The central nervous system is present in the most simple 

 condition in the Gammaridae in which the sub-oesophageal 

 ganglion is succeeded by seven thoracic (one to each segment) 

 and four abdominal ganglia, the three posterior segments being 

 supplied by a single ganglion. In other groups the number of 

 ganglia is reduced. 



The eyes are compound, but do not project beyond the contour 

 of the head.* The external cuticle forms a simple transparent 

 covering and is not, as in Isopods, divided into lenticular corneal 

 facets over the cones. In the Hyperina, which are pelagic, the 

 eyes are very large, occupying the greater part of the surface of 

 the head, and in one division of that group, the Phronimidae 

 (Fig. 307) they are differentiated into dorsally directed and later- 

 ally directed parts. The retinal pigment is present, but that of 

 the iris pigment cells is not developed in the Hyperina (Chun). 



Otosacs have been described, by Glaus, in the head of the 

 pelagic Oxycephalus, in which they contain otoliths and lie in 

 connexion with the dorsal surface of the brain, near the bases 

 of the anterior antennae though not in them. 



The olfactory hairs on the anterior antennae are referred to 

 above. Small club-shaped structures borne by the posterior 

 antennae, are known as calceoli and probably subserve some 

 sensory function. 



In the alimentary canal, the oesophagus leads into a proven- 



* The sessile condition of the eyes in Amphipods and Isopods led to their 

 classification in a group, the Edriophthalmata ( =Arthrostraca), in oppo- 

 sition to the Podophthalmata, or stalk-eyed Crustacea. 



