482 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



shell ; whose foot is a very powerful boring instrument, enabling it to 

 penetrate deeply into the sand. Here, then, we have three distinct 

 kinds of ganglionic centres ; every one of which may be doubled, or 

 repeated on the two sides of the body. First, the cephalic ganglia, a, a, 

 which are probably the sole instruments of sensation and of the consen- 

 sual movements ; these are almost invariably double, being connected 

 together by a transverse band, which arches over the oesophagus. Se- 

 cond, the pedal ganglion, b, which is usually single, in conformity with 

 the single character of the organ it supplies; but in one very rare 

 Bivalve Mollusc, the foot is double, and the pedal ganglion is double 

 also. Third, the respiratory ganglion, c, which frequently presents a 

 form that indicates a partial division into two halves, corresponding 

 with the repetition of the organs it supplies on the two sides of the 

 body. Besides these principal centres, we meet with numerous smaller 

 ones upon the nervous cords (/, f, and g, g\ which proceed from them 

 to the different parts of the general muscular envelope or mantle. 



853. Now it will be observed, that the two cephalic ganglia a, a, are 

 connected with the pedal ganglion, b, by means of a pair of trunks pro- 

 ceeding from the former to the latter ; and that they are, in like man- 

 ner, separately connected with the respiratory or branchial ganglion, c. 

 There is good reason to believe, that the pedal and branchial ganglia 

 minister to the purely reflex action of the organs they respectively sup- 

 ply ; and that they would serve this purpose as well, if altogether cut off 

 from connexion with the cephalic ganglia : whilst the cephalic, being the 

 instruments of the actions which are called forth by sensation, exert a 

 general control and direction over the movements of the animal, through 

 the medium of the trunks by which they communicate with the ganglia 

 in immediate connexion with the muscular apparatus. It is difficult, 

 however, to make satisfactory experiments upon this subject in these 

 animals, their movements being for the most part slow and feeble, and 

 their nervous system not readily accessible ; and our idea of the re- 

 spective functions of their ganglia is chiefly founded upon the distribu- 

 tion of their nerves, and upon the analogous operations of the ganglia 

 that correspond to them in other animals. 



854. In ascending through the series of the Mollusca, we find the 

 Nervous system increasing in complexity, in accordance with the gene- 

 ral organization of the body; the addition of new organs of special 

 sensation, and of new parts to be moved by muscles, involving the 

 addition of new ganglionic centres, whose functions are respectively 

 adapted to these purposes. But we find no other multiplication of 

 similar centres, than a doubling on the two sides of the body ; excepting 

 in a few cases, where the organs they supply are correspondingly mul- 

 tiplied. We have a very characteristic example of this in the arms of 

 the Cuttle-fish, which are furnished with great numbers of contractile 

 suckers, every one possessing a ganglion of its own. Here we can trace 

 very clearly the distinction between the reflex actions of each individual 

 sucker, depending upon the powers of its own ganglion ; and the move- 

 ment prompted by sensation which results from its connexion with the 

 cephalic ganglia. The nervous trunk, which proceeds to each arm, may 

 be distinctly divided into two tracts ; in one of which are contained 



