LAHELLIBUANCHIATA. " 511 



eduUs. The pedial ganglion distributes nerves in one direction to 

 the retractors, in another to the substance of the foot, and it sends 

 the nerve to the acoustic sac (s). Tiie branchial ganglions (6) send 

 off nerves (/), which diverge as they pass to the base of their re- 

 spective gills : then each gives off a large nerve (gr) which passes over 

 the adductor muscle to the hinder part of the pallial lobe, along 

 which it curves, and is continued forwards near the border of the 

 mantle until it meets and anastomoses with the corresponding nerve 

 (a) which is continued over the anterior adductor from the labial 

 ganglion. These circumpallial nerves {g, a) send off branches which , 

 form loops at the base of the tentacles from the posterior border of 

 the mantle, and, along the rest of the free border, form a circumpallial 

 plexus {h, h'), which is, also, continued along the anterior and cardinal 

 borders of the mantle, unconforraably with the circumpallial nerve.* 

 The nerves of this and other bivalves present the soft and pellucid 

 structure which is so common in the aquatic invertebrata. The mo- 

 dification of the nervous system, in other bivalve moUusca, have been 

 ably compared and homologised by Mr. Garner. f In the oyster the 

 suboesophageal loop is slender and contracted, and unconnected with 

 any other ganglion excepting the labial ones in the pedate bivalves ; 

 the suboesophageal loop is more or less lengthened, having the form 

 of a Roman arch in the Pecten, and that of the Gothic or pointed 

 arch in the Cardium and Mya ; it has for its keystone, if we may 

 pursue this analogy, the pedial ganglion. In some species this 

 ganglion is more distinctly bilobed than in others ; sometimes, as in 

 the Pholas, it is situated more superficially near the tip of the foot ; 

 in all it seems to be the centre from which the viscera derive their 

 nerves. The largest and most constant ganglions are those situated 

 upon the posterior adductor muscle, following this muscle in all its 

 varieties of position, and manifesting likewise differences in relation 

 to the branchiae, but always brought into direct communication with 

 the oral or labial ganglions. In these bivalves, as Ostrea, Cardiumy 

 Unio, Anomia, Venus, Pholas, Teredo, Solen, Mya, and Mactra, in 

 which the gills of one side are united to those of the opposite, the 

 branchial ganglia are conjoined. But in those, as Mytilus, Modiola, 

 Pecten, in which the branchia are separate, and at a distance from 

 one another, the two ganglia are distinct, and joined by a transverse 

 chord of greater or less extent. A small siphonic ganglion is deve- 

 loped at the point of confluence of the muscular respiratory tubes 

 in the bivalves which possess those accessory organs of respiration. 

 Dr. Siebold \ discovered the organ of hearing in the Cyclas Naia, 



* See the beaatifal dissections by Mr. Goadby, Nos. 613 — 619. 

 t CCCXIV. t CCCXIII. p. 49. 



