LAMELLIBRAXCHIATA. 285 



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 mollusca, have been 

 ably compared and reduced to their analogues by Mr. Garner. 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 key- 

 stone, if we may pursue this analogy, the pedial ganglion. In some 

 species this ganglion is more distinctly bilobed than in others ; some- 

 timesj 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 dif- 

 ferences in relation to the branchise, but always brought into direct 

 communication with the oral or labial ganglions. 



In these bivalves, as the Ostrea, Cardium, Unio, Anomia, Vejius, 

 PholaSi 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 the 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 developed at the point of confluence 

 of the muscular respiratory tubes in tlie bivalves which possess those 

 accessory organs of respiration. 



Dr. Siebold has recently described a small sacculus in the Cyclas 

 cornea, attached to the anterior part of the labial ganglion, and con- 

 taining a cretaceous nucleus of a crystalline structure, performing 

 remarkable oscillatory movements : this sac he regards, with much 

 probability, as a rudimental organ of hearing. The Pecten has a 

 number of small ocelli arranged around the inner margin of the , 

 mantle, and which have been regarded from the period of Poli, who 

 called the Pecten Argus, as rudimental organs of vision. Oysters 

 appear sensible of light, and close their valves when the shadow of 

 an approaching boat is thrown forward, so as to cover them before 



the beautiful dissections and drawings exhibited, were detailed by the Professor, 

 under the names of the " pre-pallial," and " post-pallial nerves ;" the " circumpallial 

 plexus," the " dorso-pallial plexus," the " visceral nerves," " branchial plexus," &c., 

 the special description of which, with figures, would, he stated, shortly appear in a 

 monograph directed to be published by the Council of the College. W.W, C. 



