384 



ARTHROPODA 



leaping motion, whence the common name, 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 



anterior thoracic feet generally bear large 

 epimeral plates (fig. 433), which extend 

 the sides of the body downwards, while 

 on the inner side delicate gills or gill sacs 

 (fig. 410, br) arise from their bases. In 

 the female brood lamella? (brl) are added 

 which enclose a brood chamber beneath 

 the body in which eggs or young are car- 

 ried. The three anterior pairs of ab- 

 dominal feet are two-branched, richly 

 haired, and serve to create currents of 

 water which pass over the gills. The 

 remaining abdominal feet, through bira- 

 mous, are shorter and stout and form 

 springing organs. The position of the 

 gills explains why the abdominal part of the heart is degenerate and only 

 the anterior thoracic portion with three pairs of ostia persists. 



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

 Live attached to other pelagic animals on which they feed; Hyperia* Phronima* 

 Sub Order II. GAMMARINA. Head much smaller; abdomen well developed; 

 mostly free swimmers. Numerous species in the sea. Gammarus* in shallow 

 water, some fluviatile; Orchestia* above tide marks. Chelura terebrans* 



FIG. 410. Cross-section of 

 kmphipodCorophium, (from Lang, 

 after Delage). bf, thoracic leg; 

 <bm, ventral nerve cord; br, 

 branchiae; brl, brood lamella; d, 

 intestine; h, heart; /, liver; ov, eggs 

 in brood chamber. 



FIG. 411. Gammarus ornatus* (from Smith) 



destroys submerged wood. Sub Order III. LffiMODIPODA. Parasitic or 

 semi-parasitic; second thoracic somite is fused to head; appendages lacking 

 from some thoracic segments, abdomen reduced. Capretta* on hydroids. 

 Cyamus ceti, parasitic on whales. 



