370 MOLLUSC 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 larvfe begin to accumulate in crowds in the 

 outer gill plates. The reproductive organs are branched 

 and large ; there are no accessory structures ; the genital 

 aperture lies on each side under that of the ureter. 



Autumn and winter months seem to be the usual periods 

 of reproduction. The ova are squeezed out of the foot, 

 and appear to be moved to the exhalent 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 spermatozoa drawn in by the 

 water currents, though it is difficult to believe that this is 

 entirely a matter of chance. Development takes place in 

 the gill cavity, which is often much distended with larvK. 



Development and Life history. 



The development of Aitodonta differs in certain details from that of 

 most bivalves, perhaps in adaptation to fresh water conditions. 

 Moreover, a temporary parasitism of the larva has complicated the 

 later stages. 



The egg cell is surrounded by a vitelline membrane, and attached to 

 the wall of the ovary by a minute stalk, the insertion of which is marked 

 on the liberated ovum by an aperture or micropyle, through which the 

 spermatozoon enters. 



Segmentation is total but unequal. A number of small clear yolkless 

 cells are rapidly divided oft" from a large yolk-containing portion, which 

 is slower in dividing. Eventually, a hollow ball of cells or blastosphere 

 results (Fig. 120). 



On the posterior dorsal region a number of large opaque cells form an 

 internally convex plate, the lieginning of the future shell sac. A pair of 

 large cells are intruded into the central cavity, and begin the mesoderm. 



On the under surface posteriorly there is a slight protrusion of ciliated 

 cells forming a ciliated disc. In front of this, at an unusually late stage, 



