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 yarietes^ a wall, and (T7tXayxvov, 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 posi- 

 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, gradually 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. c.) 

 a commissural cord proceeds forward and upward, encircling the 

 oesophagus and connecting with the cerebral ganglion of the opposite 

 side. From the lower angle of the anterior portion (PI. 9, 12, 13, 

 a. a. n.) a peripheral nerve passes directly forward into the anterior ad- 

 [Sen. Doc. No. 38.] ' 24 



