604 
Second in importance is the branchial gan- 
glion. In those Conchifers in which the two 
branchie are conjoined, the ganglion continues 
single ; where they are separate, it becomes sub- 
divi From this ganglion or these gan- 
glia nerves are derived to the branchie and 
to the respiratory siphons, when present, to 
the posterior parts of the viscera, and to the 
posterior adductor muscle, also to the mantle, 
Fig. 337. 
Nervous system of the Oyster, ( Ostrea ). 
@,a@, anterior ganglions. 6, posterior ganglion. 
e,c, branches to branchia. d, d, connecting 
trunks. ¢, transverse branch uniting anterior 
ganglia. 
In the oyster (fig. 337), the cockle (Car- 
dium), and many others, the branchie are 
united, and there is consequently but one bran- 
chial ganglion. 
In the mussel (Mytilus), in the scallop 
( Pecten ) (fig.338), &c. the branchie are sepa- 
rated and the ganglion is divided into two; 
their connection, however, is maintained by a 
commissural band. The branchial ganglia or 
ganglion are also united to the anterior or 
esophageal ones. 
Fig. 338. 
Sy 
ee 
Ss 
ANN 
X (| NS 
Nervous system of Pecten ( Scallop ). 
a, anterior ganglia. 6, branchial ganglion, c, 
pedal ganglion. d, esophagus. 
A third ganglion, the pedal, is not deve- 
loped in all the genera ; and this circumstance, 
NERVOUS SYSTEM. (Comparative Anatomy.) 
as well as its position and the distribution of 
its nerves, throws much light upon its function 
It is immediately connected with the wsopha- 
geal ganglia by two nerves; it is always deve- 
ey in the substance of the foot, generally: 
its base, and its size 1s always proportioned t& 
the power and dimensions of thatorgan, _ 
In the genus Ostracea it is entirely absent 
for the foot is wanting, and whatever locomo 
tive power is enjoyed by these animals is per 
formed by the rapid closure of their 
through the action of the adductor musc 
The arrangement of the nervous 
Conchifera is of the highest physiological it 
terest. It affords a beautiful example of 
complete analysis of the more comp icater 
nervous system of the vertebrata. e ha 
here an anterior pair of ganglia, from whic 
filaments proceed to all parts of the boc 
associated too with the ingestive faculty; the 
are connected with whatever degree of psych 
cal endowment the animal possesses and fort 
its sensorium commune; they are the soure 
of its voluntary actions. The respiratory ¢ 
gans likewise have their special centre 
branchial ganglion or ganglions, the develo 
ment of which is always proportioned to th 
of the branchie. And there is a special cent 
provided for the locomotive organ, too, wh 
development is strictly in relation with its si 
and activity, and which is absent when 
organ does not exist. And it must be observ 
that these special ganglia (respiratory and ped 
although unconnected with each ) 
municate with the cesophageal ganglia, 
Have we not here distinctly marked out 1 
Cone (the centre of volition and sensatiot 
the medulla oblongata (the respiratory centre 
and the corsbollonl (looogeaneee centre), 
they occur in the higher vertebrata? And 
the aggregate of the chords by 
cesophageal ganglia communicate with © 
pedal and branchial ones, do we not see 
analogue of at least a portion of the sp 
cord, that portion which consists of affe 
and efferent nerves to and from the bra 
The nervous system is distinctly ad 
wants of the animals and their limited ps) 
cal endowment, and the same law prev: 
throughout the scale of animals. It is no’ 
nervous system which developes the pow 
and instincts of the animal; on the cont 
these latter determine the development of 
nervous system. This is well illustrated 
comparison of the oyster and the m 
These moliusks differ only in a greate 
motive power belonging to the mussel, te 
which it possesses an organ called the 
the oyster is devoid of such an organ. 
mussel has an additional ganglion (the 
which the oyster has not, and this gang! 
not an isolated centre, but, like the bran 
ganglion, is connected by distinct fila 
with the anterior or cerebral ganglia. 
A large series of careful dissections of 
nervous system of these and other Inverte 
well displayed is a t desideratum in 
public museums. We are happy to pe 
that this want is likely to be supplied by 
2 
