Page:Popular Science Monthly Volume 10.djvu/44

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34
THE POPULAR SCIENCE MONTHLY

there are (Fig. 11) no supra-œsophageal ganglia. There is nothing, in fact, to which the term brain can be appropriately applied.

But, if we turn now to the much more active snail, we find the nervous system existing in a more developed and concentrated form. There is (Fig. 12, l) a large ganglionic mass situated over the œsophagus, each half of which receives a considerable bundle of nerve-fibres (f) from the eye (b) of the smaller side, which is situated at the tip of the larger tentacle. It also receives another bundle of nerves (k)

PSM V10 D044 Head and nervous system of the common snail.jpg
Fig. 12.—Head and Nervous System of the Common Snail.

from the small tentacle on each side, which has in all probability a tactile function. The auditory vesicles are here in a new position. They are in immediate relation with the posterior aspect of these ganglia constituting the brain, though in other gasteropods they are, as in bivalve Mollusca, found to be connected with the pedal ganglia. That gasteropods are endowed with a rudimentary sense of smell is now generally admitted by naturalists, though hitherto they have been unable to locate this endowment in any particular organ or surface-region.

The brain of the snail is connected, by means of a triple cord or commissure on each side of the œsophagus, with a still longer double ganglionic mass (m). This latter body, situated beneath the œsophagus, represents the pair of pedal and the pair of branchial ganglia of the bivalve Mollusca. Here nerves are received from the integument and given off to the muscles of the foot, while they are also received and given off from the respiratory and other organs.

In the nautilus and some other representatives of the next class, Cephalopoda, the nervous system attains a development only slightly in advance of that met with among the highest gasteropods, though in the active and predaceous cuttle-fish, and in its near ally, the octopus, we find the nervous system presenting the highest development to be met with among; the sub-kingdom Mollusca.