486 Mr. Garner on the Nervous System of Molluscous Animals. 
In the T'unicata are no lips, no foot, nor valvular muscles, and therefore the 
ganglia which supply those parts in Conchifera are absent. Their single gan- 
glion evidently presides over the functions by which the water, &c. is drawn 
in and expelled. 
The only correct description of the nerves of a Bivalve animal is that given 
by Mangili of the nerves of the dnodonta. In all Bivalves, with the exception 
of those entirely destitute of a foot, we find three ganglia, each of which is 
composed of two others: In Ostrea, which has no trace of a foot, there is no 
inferior or pedal ganglion, but only a few scattered filaments in its situation. 
The posterior ganglion is always situated at the posterior muscle between the 
branchiz. "That it is chiefly a branchial ganglion is proved by its being regu- 
lated in its disposition by the situation of those organs. Thus in Ostrea, Car- 
dium, Unio, Anomia, Venus, Pholas, Teredo, Solen, Mya, Mactra, &c., in which 
the branchiz are united together, the two ganglia which compose it form but 
one. But in Mytilus, Modiola, Pecten, &c., in which the branchiz are separated, 
and at a distance from each other, the two ganglia are more or less separated, 
always, however, united by a transverse chord. This ganglion (Tas. XXIV. 
fig. 2, 3, 4, 5, 6 & 7, A.) gives off anteriorly two nerves (a.), by which it is 
joined to the anterior or labial ganglia (B.). Besides these, the posterior 
ganglion gives nerves to the branchice (5.), large branches to the respiratory 
siphons (c.), minute visceral filaments (d.) to the posterior parts of the viscera 
(the labial ganglia sometimes giving a filament or two to the anterior parts), 
twigs (e.) to the posterior muscle, and branches to the mantle (f.). These 
ganglia and nerves are much developed in Conchifera, which, like the PAolas, 
have their branchiz and siphons large. The anterior or labial ganglia (B.) 
are never in conjunction, but always united by a transverse filament (g.), which 
arches over the mouth. In Mytilus and Modiola they are of a lengthened form, 
and situated a little behind the mouth. In the Pectines they are much more 
posterior. In Mactra, however, they have advanced forwards, and nearly 
meet over the mouth; and perhaps these Conchifera show more locomotive 
activity than any others. Besides the nerves which they receive posteriorly, 
and the connecting filament between the two, each gives off one or two 
nerves (/.) to the mantle, tentacular branches (7.), and muscular filaments (J-). 
Each ganglion likewise sends down a nerve (4.), which meets its fellow, and 
