854 FRESH-WATER BIOLOGY 



bristles and hairs which they bear. They are usually terminated by 

 two movable claws, but there may be only one, or rarely the leg may 

 end in a spine or bristle. The more active and the pelagic forms 

 have longer legs with fewer and longer spines and bristles, the less 

 active shorter, stouter legs with more thickly set and shorter bristles. 

 In some cases a number of long hairs in a close-set row on the outer 

 segments of the leg seem to aid in swimming and so are called swim- 

 ming-hairs; while in other cases curiously modified leg segments 

 and spines characterize the male and serve as accessory organs in 

 pairing (Fig. 1319). 



The genital opening is situated behind or between the epimera 

 and is usually flanked by plates which bear characteristic cup-like 

 or knob-like structures known as acetabula, the exact nature and 

 function of which is unknown. There may be in addition movable 

 flaps, which may or may not cover the acetabula, and in some cases 

 such flaps, by fusion with the genital plates, seem to have become 

 immovable. 



Between the anterior epimera is a plate, which has been termed, 

 from its form, the maxillary shield, and which is the ventral side of 

 a chitinous box called the camerostom, which encloses the mouth- 

 parts. To this are articulated the five-jointed palpi; at its anterior 

 end is the mouth-opening, through which project the stiletto-like 

 or sabre-like mandibles; and on its dorsal surface are the two 

 stigmata, leading by air-tubes into two air-sacs placed above the 

 pharynx, from which a system of tracheal tubes runs through- 

 out the body. In the forms parasitic on the fresh-water mussels 

 these tubes are lacking. The maxillary shield is frequently pro- 

 longed posteriorly into a kind of ancoral process, and the anterior 

 ventral angle of the camerostom may be produced into a sort of 

 rostrum. All these structures together are termed the capitulum. 



The sexes are separate, sexual dimorphism being a common phe- 

 nomenon, and all species lay eggs. These may, rarely, be laid free 

 in the water, but are more usually deposited singly or in mass, 

 surrounded by a gelatinous envelope, on water plants or other 

 submerged objects. The embryo undergoes considerable develop- 

 ment before escaping from the egg membranes at which time it 

 becomes an active six-legged larva (Fig. 1320). This larva after a 



