Page:Popular Science Monthly Volume 10.djvu/46

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36
THE POPULAR SCIENCE MONTHLY.

into correlated action with the jaws. The posterior portion of the sub-œsophageal mass receives nerves from, and also gives off nerves (14) to, the branchiæ and other viscera, as well as to the mantle (13, 13).

The auditory organs and their nerves are also connected with this branchial and pallial ganglion. These organs are lodged in the substance of the cartilaginous framework investing the nerve-ganglia—a structure which seems to answer to a rudimentary skull. The roots of the auditory nerves are probably principally in relation with the pallial portion of the branchio-pallial ganglion. The locomotions of these creatures are largely brought about by contractions of the pallial chamber, though these contractions of the mantle are also subservient to the respiratory function.

The share which the branchio-pallial ganglia take in bringing about and regulating the movements of the cuttle-fish would seem to explain the connection of the auditory nerves with them rather than with the homologues of the pedal ganglion, with which the auditory saccules are in relation in most other mollusks. But, whatever may be the precise explanation of the different connections of the auditory nerves in the cuttle-fish tribe, the fact remains that their connections are still away from the brain proper. They are, as in most other Mollusca and in those insects in which auditory organs are known to occur, in intimate relation with one of the principal motor centres.

This survey of some of the principal forms of the invertebrate brain, brief though it has been, should have sufficed to call attention to the following important facts and inferences:

1. That sedentary animals, though they may possess a nervous system, are often headless, and then have nothing answering to a brain.

2. That where a brain does exist, it is invariably a double organ. Its two halves may be widely separated from one another, though at other times they are fused into a single mass.

3. That the component or elementary parts of the brain in these lower animals are ganglia in connection with some of those special impressible parts or sense-organs, by means of which the animal is brought into harmony with its environment or medium.

4. That the sensory ganglia, which as an aggregate constitute the brain of invertebrate animals, are connected with one another both on the same and on opposite sides of the body, either by continuous growth or by means of commissures.

5. The size of the brain as a whole, or of its several parts, is strictly regulated by the development of the animal's special sense-organs. This is so, because, the more these impressible surfaces become elaborated and attuned to help in discriminating between numerous different external impressions, the larger are the ganglionic masses with which their nerves are in relation.

6. Of the several sense-organs and sensory ganglia whose activity