Nervous system.—The nervous system of the Gasteropoda furnishes us with the most perfect form of the heterogangliate, or as it has been less happily denominated, cyclo-gangliated type. It consists of a variable number of ganglia or nervous centres disposed in different parts of the body, but connected with each other by cords of communication, and from these ganglia the nerves appropriated to dif ferent parts proceed. Each ganglion, therefore, is a distinct brain; and were the preponderance in size to be regarded as the criterion of rela tive importance, it would not unfrequently be hard to say to which the pre-eminence is due. There is, however, as we shall soon perceive, an uniformity in the arrangement of certain masses, and a regularity in the appropriation of the nerves proceeding from them to parti cular organs, which leave us little room for 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 Stagnatis, 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 (esophagus to which the name of the brain is uoiversally 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 ease 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 esophagus; each of these gives off a number of nervous filaments, which are lost in the muscular envelope of the body ; fourth ganglion joined to the brain by two cords is found under the fleshy mass of the mouth ; this supplies the esophagus, 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 branchial 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 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-cesophageal gan glion connected with it by two cords embracing the (esophageal 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-esopha 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 cesophagus 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 esophagus will in some circumstances permit the mass of the mouth to pass entirely through it, so that sometimes the brain rests upon the ceso phagus, and at others is placed upon the in verted lips.