400 PHYLUM MOLLUSCA. 
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 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 larve feed for some time on mucus 
secreted by the gill. 
Development and life history.—The development of Asodonta 
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 establishes 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 sed 
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 
