ANATOMY AND PHYSIOLOGY OF ANODONTA FLUVIATILIS. 185 
THE NERVOUS SYSTEM. 
(Plates 9, 12, 13.) 
The nervous system consists of nerve centers or ganglia, connected 
by nerves designated commissural cords or commissures, and nerves 
proceeding from the nerve centers to different portions of the body, 
and known as peripheral nerves. 
The nerve ganglia are designated as the anterior or cerebral, the 
pedal, and the posterior or parieto-splanchnic —a compound word 
derived from the Latin paretes, a wall, and onlayyvor, the intes- 
tine —and are so named because nerves of the mantle in part, the 
gills and of the viscera, were supposed to proceed from this ganglion ; 
but the pedal ganglia furnish the nerves of the viscera, and the cere- 
bral ganglia furnish nerves to a large portion of the mantle. The 
name being both unwieldy and calculated to mislead, I shall use the 
term posterior both as being simpler and precisely defining the posts 
tion of the ganglion. 
On Plate 9 is a figure showing the nerves in position, on Plate 13 
the nerve centers enlarged, and on Plate 12 the nerves dissected out 
and enlarged twice. The same letters apply to each of the plates. 
The different nerves will, however, be more clearly distinguished on 
Plate 12. 
The anterior or cerebral ganglia (Pl. 9, 12, 13, c. g.) are two in number, 
and are situated one on each side of the animal, just back of the 
lower portion of the anterior adductor muscle and between that mus- 
cle and the protractor pedis, very near the surface, so that when the 
shell is opened by cutting the muscles close to the shell, the ganglia 
are generally exposed. They are wider than thick. The anterior por- 
tion being the widest, graduaily narrowing to the posterior portion, 
which is about two-thirds the width of the anterior; the length is a 
little more than the width of the anterior portion. The measurements 
in one specimen are as follows: width of anterior portion, one 
millimetre ; posterior portion, two-thirds of one millimetre; length, 
one and one-half millimetres ; thickness, a little less than two-thirds 
of one millimetre. On account of the two nerves proceeding from 
the upper portion, and the two from the lower portion, the ganglion 
presents an indistinct bilobate appearance. 
From the upper angle of the anterior portion (PI. 9, 12, 13, c. ¢.) 
a commissural cord proceeds forward and upward, encircling the 
cesophagus and connecting with the cerebral ganglion of the opposite 
side. From the lower angle of the anterior portion (Pl. 9, 12, 13, 
a. a. 0.) a peripheral nerve passes directly forward into the anterior ad- 
[Sen. Doc. No. 38. ] 24 
