Page:Natural History, Mollusca.djvu/15

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MOLLUSCA.

ever, first indicate those which they possess in common, and by which they are naturally grouped together.

The nervous system demands our first attention. Instead of a great mass of nervous substance accumulated in one place, and a lengthened spinal cord proceeding from it, giving out threads to all parts of the body, as in the Vertebrata, we find the nervous centres numerous, unsymmetrical and disposed in various parts of the system, no one having so decided a predominance over the others in bulk, as to merit the appellation of a brain. There is, however, one mass larger than the rest, which is always placed either above the gullet (œsophagus), or encircling it, in the form of a thickened ring; and from this the nerves that supply the organs of sense invariably originate. This mass, or ganglion, must undoubtedly be regarded as the representative of the brain; for in the most highly organized animals of the Division, the Squids and Cuttles (Cephalopoda), this encirling mass is enclosed and defended by a case of cartilage, the lingering rudiment of a bony skull.

The accompanying engraving, which is copied from Professor Grant's "Outlines of Comparative Anatomy," will give the reader an idea of the system of nerves and ganglia, with some of the other organs, as they appear in Bulla lignaria, a large and handsome shelled Mollusk found on the British coasts.

In the above figure the chief ganglion forms a ring, (marked ee,); anterior to this there is a small ganglion, not seen, because situated below the bulb of the gullet (a), just before the insertions of its diverging muscular bands (c), and behind the