400 PH YL UM MOLL USCA. 



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 larvae 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 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 spermatozoa 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 Aiiodonta 

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

 tion 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 off from a large yolk-containing portion, which 

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

 results (Fig. 215). 



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

 an internally convex plate, the beginning 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, 

 an invagination established the archenteron, and the embryo becomes a 

 gastrula (see Fig. 215). 



The shell-sac forms an embryonic shell, and many of the mesoderm 

 cells combine in an adductor muscle. The mouth of the gastrula closes, 

 and a definite mouth is subsequently formed by an ectodermic invagina- 

 tion. Gradually a larva peculiar to fresh-water mussels, and known as 

 a Glochidium, is built up. 



The Glochidium has two triangular, delicate, and porous shell valves, 

 each with a spiny incurved tooth on its free edge. The valves clap 

 together by the action of the adductor muscle. The mantle lobes are 

 very small, and their margins bear on each side three or four patches of 



