far the greater number are aquatic, and breathe by means of gills. But being all of them, as Prof. Owen says, "endowed with power to attain, subdue, and devour organic matter, dead and living," we find their nervous system not only better developed, more complex and concentrated, but also in relation with more highly-evolved organs of special sense and exploration. It offers considerable variations in its general arrangement, especially as regards relative positions of ganglia, though these modifications are, to a great extent, referable to differences in the outward configuration of the body.
Some of the differences in external form which are to be met with among gasteropods are well illustrated by the limpet or the chiton, as compared with the snail. Here differences in form coexist with differences in habit, so that we almost necessarily meet with notable variations in the disposition of the principal parts of the nervous system.
In the limpet we find that the two small cerebral ganglia (Fig. 10, a) are widely separated from one another, and lie at the side of the œsophagus. Each receives a rather large nerve from one of the tentacles, and a smaller optic nerve. A commissure connects these cerebral ganglia above the œsophagus with one another, while each of them is also in relation by means of two descending commissures with a series of four connected ganglia forming a transversely-arranged row beneath the œsophagus. Of these the two median ganglia (B) correspond with the pedal, while the two external (C) correspond with the branchial ganglia, though they are here separated from one another by an immensely wide interval.
|Fig. 10.—Nervous System of the Common Limpet.||Fig. 11.—Nervous System of Chilon marmoratus.|
However small and undeveloped the duplex brain of the limpet may be, this organ exists in an even more rudimentary state in some other gasteropods. Thus, in the chiton, which is a close ally of the limpet, and about the most simply organized of all the gasteropods, there are neither tentacles nor eyes, and, as a consequence of this,