Chap. XVI.] THE FRESH-WATER MUSSEL. 317 



The Nervous System (Fig. 93). The nervous system of the 

 mussel consists of three pairs of ganglia with their commissures. 

 On the dorsal side of the mouth there are two somewhat 

 triangular cerebral ganglia (c. g.) united by a short commissure 

 which passes over the mouth. From each of these ganglia, 

 besides local branches c.o.c. 



to the labial palps and 



ci.r.i 



the neighbouring mus- 

 cles, there passes down- 

 wards and slightly back- , 



6 * a.acl.- 



wards a cereoro - pedal 

 commissure (c. p. c.\ end- c f- c -' 

 ing in the pedal ganglion 

 (p. g.). The two pedal 



ganglia are Orange Fia - 93. MUSSEL : NERVOUS SYSTEM. 



bodies applied to each d. Anterior adductor muscle, a. r. m. Anterior 



fVi * tVia rnirl linp retractor muscle, c. g. Cerebral ganglion, c. o. c. 



me ' Cerebro-olfactory commissure, c. p. c. Cerebro-pedal 



Each gives Off a branch commissure. /. Foot. ol. g. Olfactory ganglion, p. ad. 



tn thp Tninntp anrlitnrv Posterior adductor muscle, p. c. Pericardial chamber. 



' 01 T Pmfft Pedal ganglion, p.p. Protractor pedis muscle. 



Organ, and branches to p.r.m. Posterior retractor muscle, re. Renal organ. 



the neighbouring parts. 



From each cerebral ganglion there also passes off, upwards and 

 backwards, a cerebro-olfactory commissure (c. o. c. ) (cerebro-parieto- 

 splanchnic of some authors), which is readily traced in the 

 region of the renal organ. It proceeds to an olfactory ganglion 

 (ol. g.) (parieto-splanchnic of some authors), which lies near the 

 surface beneath the posterior adductor muscle. 



Eecent observations seem to show that there are two classes 

 of nerve-fibres connected with the great adductor muscles the 

 one class motor, giving rise to contraction, the other class 

 inhibitory, producing relaxation, when mainly by the elasticity 

 of the ligament the shell gapes. The motor nerves for each 

 muscle spring from the ganglion next it ; the inhibitory fibres, 

 on the other hand, are stated to take their origin entirely from 

 the cerebral ganglia. 



Special Senses. We know little about the special senses in 



