Mr. Garner on the Nervous System of Molluscous Anlnuds. 487 



forms a ganglion (C.) in the substance of the foot, giving many branches to 

 this organ. This ganglion in Bivalves is never divided : that it chiefly belongs 

 to the foot, and not to the viscera, is proved by its being regulated in size by 

 the development of tlie foot, and being absent when that organ has disap- 

 peared. The author could never trace any filaments from it to the viscera. 

 It is large in Solen, Mactra, Unio, &c., small in Pecieii, Mya and Jnomia, and 

 absent in Ostrea. It is generally, as in Mactra, situated between the muscular 

 tissue and the viscera; more forwards in Cardimn echinafum; in Pecten at 

 the anterior part of the base ; in Pholas superficially at the point. 



In Conchifera then the mouth is surrounded by a ring, of which the lower 

 part is double. This ring is, however, very wide, other organs besides the 

 mouth being within it. Generally the nervous system is symmetrical ; but 

 when the animal, as Ostrea, is inequivalve, the nerves going to the branchiae 

 and mantle in the deeper valve are lengthened and disarranged. In Anomia 

 the anterior ganglia are displaced, and the inferior become lateral, from the 

 change in the position of the mantle and foot. The ganglia of Conchifera 

 are of an orange colour; in those, however, of which the tissues are trans- 

 parent, they are whiter. 



The anterior and posterior ganglia are figured by Poll in many Bivalves ; in 

 no instance has he described the inferior one. It is well known that he 

 considered these nerves to be lacteals, and the ganglia receptacula chyli, from 

 the possibility of injecting their sheaths. 



That the anterior ganglia are the cerebral or sentient lobes of the animal 

 appears from this, that the other pairs communicate with them, and not with 

 each other. The separate ganglia of each pair are conjoined that their action 

 may be consentaneous. The pedal, from its supplying the foot, may be cor- 

 rectly termed the ganglion of locomotion ; whilst the posterior, supplying the 

 branchise and siphons, may be termed respiratory : but as each pair supplies 

 likewise other parts, their functions cannot be purely so limited, though it is 

 probable that the subordinate function is derived from twigs they receive from 

 the others, incorporated in the connecting nerves. The anterior ganglia in 

 the Pecten (fig. 5, B.) are seen to be composed of two portions, one coloured 

 and soft, the other fibrous, composed of filaments passing through the gan- 

 glia. 



