DEVELOPMENT AND LIFE HISTORY. 353 



of united and reflected gill filaments. On the gills there are often 

 parasitic mites (Atax). 



Excretory system. — The paired kidney, which used to 

 be called the " organ of Bojanus," lies beneath the floor of 

 the pericardium. Each half is a nephridium bent upon 

 itself, with the loop posterior, the two ends anterior. The 

 lower part of this bent tube is the true kidney ; it is dark 

 in colour, spongy in texture, and excretes guanin and other 

 nitrogenous waste from the blood which passes through it. 

 It has an internal opening into the pericardium, which thus 

 communicates indirectly with the exterior. The upper part 

 of the bent tube, lying next the floor of the pericardium, is 

 merely a ureter. It conveys waste products from the 

 glandular part to the exterior, and opens anteriorly just 

 under the place where the inner gill-plate is attached to the 

 visceral mass. As already mentioned, the "pericardial 

 glands " probably aid in excretion, and possibly the same 

 may be said of the mantle. 



The reproductive organs. — These lie in the upper part 

 of the foot, adjacent to the digestive gland. Ovaries and 

 testes occur in different animals, and the two sexes are 

 distinguishable, though not very distinctly, by the greater 

 whiteness of the testes and by slight differences in the shells. 

 The females are easily known when the larva? begin to 

 accumulate in crowds in the outer gill-plates. The repro- 

 ductive organs are branched and large ; there are no 

 accessory structures ; the genital aperture lies on each side 

 under that of the ureter. 



The autumn and winter months seem to be the usual 

 periods for reproduction. The ova pass from the ovaries 

 in the foot, and appear to be moved to the exhalant region, 

 whence, however, they do not escape, but are crowded 

 backward till they pass into the cavity of the outer gill- 

 plate. At some stage they are fertilised by sperma- 

 tozoa drawn in by the water currents, though it is difficult 

 to believe that this is entirely a matter of chance. 

 Development takes place within the external gill-plate, 

 and the larvae feed for some time on mucus secreted by 

 the gill. 



Development and life history. — The development of Anodonta 

 differs in certain details from that of most bivalves, perhaps in adapta- 



23 



