THE ANATOMY OF THE RIVER-MU SSEL . 
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quently pass into the anterior part of the sac formed by the 
laminae of the inner gill. They then travel along the dorsal 
region of this sac till they reach the “ cloaca,” or terminal 
part of the anal chamber ; and thence, the shell being closed, 
and being themselves crowded on a tergo , of necessity pass into 
the external gill-sac, where, as in a kind of marsupial pouch, 
they pass through the rest of their period of incubation. Here 
the young, which were described by Bathke as parasites, under 
the name of Glochidium , are hatched, and attach themselves 
by means of a provisional byssus. The following points in the 
development of the embryo may be not uninteresting. 
The yolk-mass divides into two portions, in each of which is 
developed a distinct mouth, intestine, and heart; these are 
ultimately fused together, and thus it comes that the origin- 
ally double rectum is included within a single ventricle formed 
by the blending of once separate halves. 
At a certain period, a rotation — about seven or eight times 
in a minute — can be seen of the embryo in the ovum. This 
may be due either to ciliary action, or as Dr. Eansom has shown 
to be the case with the ova of osseous fishes, to the influence 
of oxygen in the surrounding medium. 
Nerve System. — There are three pairs of nerve centres, or 
“ ganglia ” (distinguishable without much difficulty by being 
coloured with red and yellow pigment), which are brought into 
relation with each other by connecting or “commissural” nerve 
cords, and themselves also give off nerves, which may be con- 
veniently termed “ peripheral,” to various organs. 
The most anterior pair of ganglia — the “ labial ” (fig. 7, l g) 
— are situated, one on either side of the mouth, upon the 
tendon of the anterior retractor muscle ; the intermediate pair 
— the “pedal” — (fig. 7, p g) are closely approximated and 
lodged in the foot mass about half way down, and not far from 
its ventral margin, while the most posterior pair — the branchial 
or “ parieto-splanchnic ” (fig. 7, b g ) — which are fused together, 
but are more or less bilobed, rest upon the ventral surface of 
the posterior adductor, and may be easily seen after cutting 
through the line of junction of the two inner gill laminae. 
The nerves are as follows : — a. Commissural. A cord pass- 
ing above the mouth from one labial ganglion to the other; 
next, a pair of nerves to the pedal ganglion from the labial 
ganglion of either side ; and, lastly, a pair which run straight 
backward through the glandular portion of the organ of Boja- 
nus on either side to the parieto-splanchnic ganglion. — ft. Peri- 
pheral. From the labial ganglion of either side is given 
off a branch anteriorly to the anterior adductor, and laterally 
to the anterior region of the mantle-lobe ; the pedal ganglion, 
as its name implies, sends nerves to the foot, and also to the 
