NERVOUS SYSTEM OF THE LOWER MOLLUSCA. 



251 



in the neighbourhood of the posterior adductor muscle ; and its branches are 



distributed to that muscle, to the mantle, 

 through which the water is introduced 

 and carried off. But we find another 

 ganglion, or rather pair of ganglia, a, 

 a, situated near the front of the body, 

 either upon the o?sophagus, or at its 

 sides ; these ganglia are connected 

 with the very sensitive tentacula 

 which guard the mouth ; and they 

 may be regarded as presenting the 

 first approach, both in position and 

 functions, to the brain of higher ani- 

 mals. In the Oyster, and others of 

 the lower Conchifera which have no 

 foot, which is a muscular tongue- 

 like organ, we find an additional 

 ganglion (b) connected with it. This 

 is the case in the Solen, or animal of 

 the Razor-shell ; whose foot is a very 

 powerful boring instrument, enabling 

 it to penetrate deeply into the sand. 

 Here, then, we have three distinct 

 kinds of ganglionic centres ; every one 

 of which may be doubled, or repeated 

 on the two sides of the body. First, 

 the cephalic ganglia, a, a, which are 

 probably the sole instruments of sen- 

 sation and of consensual movements; 

 as well as of whatever voluntary 

 power the animal may possess : these 

 are almost invariably double, being 

 connected together by a transverse 

 band, which arches over the oesopha- 

 gus. Second, the pedal ganglion, b, 

 which is usually single, in conformity 

 with the single character of the organ 

 it supplies ; but in one very rare Bi- 

 valve Mollusk, the foot is double, and 

 the pedal ganglion is double also. 

 Third, the respiratory ganglion, c, 

 which frequently presents a form that 

 indicates a partial division into two 

 halves, corresponding with the repe- 

 tition of the organs it supplies, on the 

 two sides of the body. Besides these 

 principal centres, we meet with nu- 

 merous smaller ones upon the nervous 



to the gills, and to the siphons 



Fiz. 124. 



Nervous system of Solen; a, a. cephalic ganglia, 

 connected by a transverse band passing over the 

 oesophagus ; b, pedal ganglion, the branches of 

 which are distributed to the powerful muscular 

 foot; c, branchial ganglion, the branches of which 

 proceed to the gills g, the siphons t, (', and other 

 parts ; h, anus : e, trunks connecting cephalic and 

 branchial ganglia; f.f.f.f, minute ganglia on the 

 branches distributed to the mantle. 



cords (f,f, and g, g], which proceed from them to the different parts of the 

 general muscular envelope or mantle. 



318. Now it will be observed, that the two cephalic ganglia a, a, are con- 

 nected with the pedal ganglion, 6, by means of a pair of trunks, e, proceeding 

 from the former to the latter ; and that they are, in like manner, separately 

 connected with the respiratory or branchial ganglion c. It is found, upon 



