ANATOMY OF THE LAMELLIBRANCHIATA. 129 



of the flat valve, and sends others to the foot and mantle. ■ 



NERVOUS SYSTEM. 



Tn more than twenty genera, examined for the purpose, the 

 neivous system has presented few differences ; and these dif- 

 ferences arise from the greater or less developement of certain 

 parts, and the greater or less distance at which the organs are 

 from each other. MangiliV description of the nerves of the 

 Anadonta is the only correct account of this system in these 

 animals. Poli mistook the nerves for lacteals, as their sheaths 

 are readily injected when the nervous pulp is softened by in- 

 cipient putrescence. Cuvier was not aware of the existence 

 of the pedal ganglion. Blainville considers the labial gan- 

 glia to be infra-cesophageal, and does not find the filament con- 

 necting the labial and pedal ganglia. When a foot is present 

 there are three ganglia, or pairs of ganglia ; when absent, but 

 two. These ganglia are of an orange colour externally, and 

 white within. Two ganglia are situated at the mouth, more 

 or less removed from each other, but always connected by a 

 supra-oesophageal nerve ; they are sometimes on a level with 

 or before the mouth, sometimes behind it. They give off on 

 each side filaments to the anterior muscle, tentacles, lips, and 

 anterior part of the mantle. Each ganglion likewise gives 

 off a twig, going to the posterior ganglia, which are situated 

 between the branchiae, on the posterior muscle. These are 

 united into one, when the branchiae are united medianly, as 

 in Mactra, Mya, Solen, &c. ; but at a distance from each 

 other when the branchiae are remote ; 3 but when so, are al- 

 ways connected by a transverse nerve as in Modiola, Avicula, 

 Lithodomus, Area, &c. These ganglia give nerves to the 

 branchiae, syphons, viscera, posterior muscle, mantle, &c. 

 The anterior ganglia also give off two twigs, which enter the 

 foot and unite into a double ganglion, from which that organ 

 is supplied with nerves. The posterior and pedal ganglia are 

 totally unconnected with each other. The mouth then is 

 surrounded by a wide ring, of which the part posterior to the 

 situation of the anterior ganglia upon it is double. 



(7b be continued.) 



1 See Reaumur, 'Du mouvement de quelques coquillages.' Mem. Acad. 

 Sciences, par 1710. Des differentes manieres dont plusieurs animaux s'at- 

 tachent, id. 171. 



2 Archives fur Physiol, b. 9. 



3 In Venus, where the ganglia are united, the branchiae, though divided 

 medianly are not remote. 



