GASTEROPODA. 



393 



hesitation upon this point ; before, however, 

 entering upon a more detailed account, we 

 will offer a few general observations upon this 

 system, applicable to the whole class. The 

 nervous centres are obviously of a different 

 nature from the cords by means of which they 

 are connected into one system, and from the 

 nerves arising from them ; the nervous mass 

 of the ganglion itself is generally granular in 

 its appearance, whilst the texture of the nerves 

 is homogeneous and smooth ; the distinction 

 is, however, in a few instances, rendered still 

 more remarkable by a striking difference in 

 colour; thus in Aplysia, whilst the nerves are 

 of a pure white, the ganglionic centres are of 

 a beautiful red tint; the same circumstance is 

 met with in the Bulimus Stagna'is, and has 

 also been remarked in many of the conchi- 

 ferous Mollusca. A second peculiarity may 

 be noticed in the mode in which the nerves 

 and ganglia are invested with a neurilema or 

 sheath, so loosely connected with them that it 

 may be inflated or injected with great facility, 

 and for this reason the nerves have been mis- 

 taken for vessels by some authors. 



As an example of the most perfectly dis- 

 persed arrangement of the nervous centres we 

 shall select Aplysia, in which the ganglia are 

 more numerous than in the generality of the 

 Gasteropod Mollusks. In this animal we find 

 a ganglion placed above the oesophagus to 

 which the name of the brain is universally 

 allowed, not so much on account of its size 

 as because throughout the class it constantly 

 occupies the same position, and as invariably 

 supplies those nerves which are distributed to 

 the most important organs of sense ; in this 

 case its branches run to the muscles of the 

 head and to the male organ of generation ; it 

 likewise sends on either side a large branch to 

 each of the great tentacles, which as they 

 approach those organs give origin to the optic 

 nerves. 



On each side of the oesophagus is found 

 another ganglion equalling the brain in size, 

 and constituting two other nervous centres, 

 which are united to each other and to the brain 

 by cords so disposed as to form a collar around 

 the oesophagus; each of these gives off a 

 number of nervous filaments, which are lost 

 in the muscular envelope of the body ; a 

 fourth ganglion joined to the brain by two 

 cords is found under the fleshy mass of the 

 mouth ; this supplies the oesophagus, the 

 muscles of the mouth, and the salivary glands. 

 At a considerable distance from these, and 

 placed near the posterior portion of the body 

 in the vicinity of the female generative organs 

 and the respiratory apparatus, is a fifth gan- 

 glion communicating with the second and third 

 by means of two long nerves, and giving 

 branches to the liver, the alimentary canal, 

 the female generative system, as also to the 

 branchiae and the muscles of the operculum. 

 From this account it will be seen that none of 

 these ganglia can be said to preside exclusively 

 over any particular apparatus, branches from 

 each being distributed to very different struc- 

 tures ; but yet, speaking generally, there ap- 



VOL. II. 



pears to be some reason for classifying their 

 functions. Thus the brain is exclusively the 

 centre of the principal senses : the two great 

 lateral ganglia supply the bulk of the muscu- 

 lar system ; the sub-oral ganglion is particularly 

 subservient to mastication and deglutition, and 

 the fifth or posterior nucleus being almost 

 entirely appropriated to the supply of the 

 digestive, respiratory, circulatory, and gene- 

 rative viscera, might be regarded as analogous 

 to the sympathetic. There are, however, but 

 few of the Gasteropoda in which the ganglia 

 are so distinct in position and function as in 

 Aplysia. In the inoperculate pulmonary Gas- 

 teropods, as in the Snail and Slug, the nervous 

 centres are only two in number, namely, the 

 brain, placed in its usual position above the 

 oesophagus, and a large sub-oesophageal gan- 

 glion connected with it by two cords embracing 

 the oesophageal tube. The brain in this case 

 supplies nerves to the muscles of the mouth 

 and lips, as well as to the skin in their vici- 

 nity ; it likewise furnishes the nerves of touch 

 and of vision, besides those distributed to the 

 generative organs, and from the sub-oesopha- 

 geal ganglion, which fully equals the brain in 

 size, arise those nerves which supply the 

 muscles of the body and the viscera. There is, 

 however, placed under the oesophagus a very 

 minute nervous mass, which from the con- 

 stancy of its occurrence is worthy of notice ; 

 it is formed by the union of two minute nerves 

 arising from the brain, and the little filaments 

 which it gives off are lost in the oesophagus 

 itself. 



One remarkable circumstance may be men- 

 tioned as being probably peculiar to the class 

 under consideration, namely, the changes of 

 position to which their nervous centres are 

 subject ; obeying the movements of the mass 

 of the mouth, with which they are inti- 

 mately connected, they are pulled backwards 

 and forwards by the muscles serving for the 

 protrusion and retraction of the oral appa- 

 ratus, and are thus constantly changing their 

 relations with the surrounding parts. In the 

 Snail it would seem that the great size of the 

 nervous collar which embraces the oesophagus 

 will in some circumstances permit the mass 

 of the mouth to pass entirely through it, so 

 that sometimes the brain rests upon the oeso- 

 phagus, and at others is placed upon the in- 

 verted lips. 



In most of the Pectinibranchiata, the brain 

 consists of two ganglia united by a transverse 

 cord ; from these two centres arise the principal 

 nerves, two of which unite to form a small 

 ganglion beneath the oesophagus, from which 

 that tube derives its peculiar supply. 



It is in the Nudibranchiate division, how- 

 ever, that the nervous centres exist in their 

 most concentrated form, and in these it is 

 doubtful whether there are any ganglia, except 

 the large supra-cesophageal brain. We may 

 take Tritonia as an example of this form of the 

 nervous system. In this beautiful Gasteropod 

 the brain consists of four tubercles placed 

 across the commencement of the oesophagus, 

 the nervous collar being completed by a simple 



2 D 



