C04 



NERVOUS SYSTEM. 



Second in importance is the branchial gan- 

 glion. In those Conchifers in which the two 

 branchiae are conjoined, the ganglion continues 

 single; where they are separate, it becomes sub- 

 divided. From this ganglion or these gan- 

 glia nerves are derived to the branchiae and 

 to the respiratory siphons, when present, to 

 the posterior parts of the viscera, and to the 

 posterior adductor muscle, also to the mantle. 



Fig. 337. 



Nervous system of the Oyster, (Ostrea). 

 a, a, anterior ganglions, b, posterior ganglion. 

 c, c, branches to branchia. d, d, connecting 

 trunks, e, transverse branch uniting anterior 

 ganglia. 



In the oyster (Jig. 337), the cockle (Car- 

 dium), and many others, the branchiae are 

 united, and there is consequently but one bran- 

 chial ganglion. 



In the mussel (Mytilm), in the scallop 

 (Pecten) (Jig. 338), &c. the branchiae are sepa- 

 rated and the ganglion is .divided into two; 

 their connection, however, is maintained by a 

 commissural band. The branchial ganglia or 

 ganglion are also united to the anterior or 

 cesophageal ones. 



Fig. 338. 



Nervous system of Pecten (Scallop). 

 a, anterior ganglia. b, branchial ganglion, c, 

 pedal ganglion, d, oesophagus. 



A third ganglion, the pedal, is not deve- 

 loped in all the genera ; and this circumstance, 



(COMPARATIVE ANATOMY.) 



as well as its position and the distribution of 

 its nerves, throws much light upon its function. 

 It is immediately connected with the cesopha- 

 geal ganglia by two nerves; it is always deve- 

 loped in the substance of the foot, generally at 

 its base, and its size is always proportioned to 

 the power and dimensions of that organ. 



In the genus Ostracea it is entirely absent; 

 for the foot is wanting, and whatever locomo- 

 tive power is enjoyed by these animals is per- 

 formed by the rapid closure of their shells 

 through the action of the adductor muscle. 



The arrangement of the nervous system in 

 Conchifera is of the highest physiological in- 

 terest. It affords a beautiful example of a 

 complete analysis of the more complicated 

 nervous system of the vertebrata. We have 

 here an anterior pair of ganglia, from which 

 filaments proceed to all parts of the body, 

 associated too with the ingestive faculty ; they 

 are connected with whatever degree of psychi- 

 cal endowment the animal possesses and form 

 its sensorium commune; they are the source 

 of its voluntary actions. The respiratory or- 

 gans likewise have their special centre in the 

 branchial ganglion or ganglions, the develop- 

 ment of which is always proportioned to that 

 of the branchiae. And there is a special centre 

 provided for the locomotive organ, too, whose 

 development is strictly in relation with its size 

 and activity, and which is absent when that 

 organ does not exist. And it must be observed 

 that these special ganglia (respiratory and pedal), 

 although unconnected with each other, com- 

 municate with the cesophageal ganglia. 



Have we not here distinctly marked out the 

 cerebrum (the centre of volition and sensation), 

 the medulla oblongata (the respiratory centre), 

 and the cerebellum (locomotive centre), as 

 they occur in the higher vertebrata ? And in 

 the aggregate of the chords by which the 

 cesophageal ganglia communicate with the 

 pedal and branchial ones, do we not see the 

 analogue of at least a portion of the spinal 

 cord, that portion which consists of afferent 

 and efferent nerves to and from the brain ? 

 The nervous system is distinctly adapted to the 

 wants of the animals and their limited psychi- 

 cal endowment, and the same law prevails 

 throughout the scale of animals. It is not the 

 nervous system which developes the powers 

 and instincts of the animal ; on the contrary, 

 these latter determine the development of the 

 nervous system. This is well illustrated by a 

 comparison of the oyster and the mussel. 

 These moliusks differ only in a greater loco- 

 motive power belonging to the mussel, to effect 

 which it possesses an organ called the foot; 

 the oyster is devoid of such an organ. The 

 mussel has an additional ganglion (the pedal) 

 which the oyster has not, and this ganglion is 

 not an isolated centre, but, like the branchial 

 ganglion, is connected by distinct filaments 

 with the anterior or cerebral ganglia. 



A large series of careful dissections of the 

 nervous system of these and other Invertebrata 

 well displayed is a great desideratum in our 

 public museums. We are happy to perceive 

 that this want is likely to be supplied by the 



