486 Mr. Garner on the Nervous System of Molluscous Animals. 



In the Tunicata are no lips, no foot, nor valvular muscles, and therefore the 

 ganglia which supply those parts in Conchifera are absent. Their single gan- 

 glion evidently presides over the functions by which the water, &c. is drawn 

 in and expelled. 



The only correct description of the nerves of a Bivalve animal is that given 

 by Mangili of the nerves of the Anodonta. In all Bivalves, with the exception 

 of those entirely destitute of a foot, we find three ganglia, each of which is 

 composed of two others. In Ostrea, which has no trace of a foot, there is no 

 inferior or pedal ganglion, but only a few scattered filaments in its situation. 

 The posterior ganglion is always situated at the posterior muscle between the 

 braiichiise. That it is chiefly a branchial ganglion is proved by its being regu- 

 lated in its disposition by the situation of those organs. Thus in Ostrea, Car- 

 diiim, Unio, Anomia, Feniis, Pholas, Teredo, Solen, My a, Mactra, &c., in which 

 the branchiae are united together, the two ganglia which compose it form but 

 one. But in Mytilus, Modiola, Pecten, &c., in which the branchiae are separated, 

 and at a distance from each other, the two ganglia are more or less separated, 

 always, however, imited by a transverse chord. This ganglion (Tab. XXIV. 

 fig. 2, 3, 4, 5, 6 & 7, A.) gives off anteriorly two nerves («.), by which it is 

 joined to the anterior or labial ganglia (B.). Besides these, the posterior 

 ganglion gives nerves to the branchiae (i.), large branches to the respiratory 

 siphons (c), minute visceral filaments («?.) to the posterior parts of the viscera 

 (the labial ganglia sometimes giving a filament or two to the anterior parts), 

 twigs (e.) to the posterior muscle, and branches to the mantle (/.). These 

 ganglia and nerves are much developed in Conchifera, which, like the Pholas, 

 have their branchiae and siphons large. The anterior or labial ganglia (B.) 

 are never in conjunction, but always united by a transverse filament [g.), which 

 arches over the mouth. In Mytilus and Modiola they are of a lengthened form, 

 and situated a little behind the mouth. In the Pectines they are much more 

 posterior. In Mactra, however, they have advanced forwards, and nearly 

 meet over the mouth ; and perhaps these Conchifera show more locomotive 

 activity than any others. Besides the nerves which they receive posteriorly, 

 and the connecting filament between the two, each gives off one or two 

 nerves (Ji.) to the mantle, tentacular branches (<.), and muscular filaments {j.). 

 Each ganglion likewise sends down a nerve {k.), whicii meets its fellow, and 



