L CRUSTACEA: AMPHIPODA. 



439 



beach fleas. In swimming the abdomen is alternately bent against 

 the breast and then forcibly straightened. 



The body is usually strongly compressed from side to side. 

 The thoracic feet generally bear large epineural plates (fig. 433), 

 which extend the sides of the body 

 downward, while on the inner side 

 delicate gills or gill sacs (fig. 449, 

 br) arise from their bases. In the 

 female brood lamellae (brl) are 

 added broad chitinous plates 

 which enclose a brood chamber 

 beneath the body in which eggs or 

 young are carried. 'The three an- 

 terior pairs of abdominal feet are 

 two-branched, richly haired, and 

 serve to create currents of water Fl - 449 -Cross-section of Amphipod 



(Corophium). (From Lang, after De- 



which pass forward over the gills. 



The remaining abdominal feet, 



though biramous, are short and 



stout and form springing organs 



explains why the abdominal part of the heart is degenerate and 



only the anterior thoracic portion with three pairs of ostia persists. 



thoracic leg; bm, ventral 

 chtae; brl, brood 



lage.) b/, 



nerve cord; br, branc 



lamella; d< intestine; 7i, heart; I, liver: 



ov, eggs in brood chamber. 



The position of the gills 



Sub Order I. HYPERINA. Large head and eyes; strong prehensile feet. 

 Live attached to other pelagic animals on which they feed. Hyperia 

 medusarum * lives on the jelly fish Cyanea; Plironima,* warmer seas. 



Sub Order II. GAMMARINA. Head much smaller; abdomen well devel- 

 oped; are mostly free swimmers. Numerous species in the sea. Cfam- 



Fig. 450. Gammarus ornatus.* (From Smith.) 



marus * occurs in shallow water, some being fluviatile; Orchestia * above 



tide marks. Chelura terebrans * destroys piles and other submerged wood. 



Sub Order III. L^EMODIPODA. Parasitic or semi-parasitic forms in 



which the first (second) somite is fused to the head; appendages are lacking 



