Page:Popular Science Monthly Volume 9.djvu/733

Rh a single lower ganglion; and, as a consequence of the coalescence of the two lower halves, we have, instead of the two lateral cords of the Nemertidæ, a double ventral nervous cord traversing the whole length of the body. There are no distinct ocelli in the earthworm. The body is composed of a multitude of ring-like segments, each of which is provided with lateral setæ, which are called into play during the subterranean locomotions of the animal.

The double ventral cord has a fibrous structure along its upper surface, while below there is an irregular stratum of ganglion-cells. These cells are more abundant about the centre of each body-segment, and their aggregation gives rise to a series of rudimentary ganglia in these situations. From, every one of the ganglionic swellings two nerves are given off on each side, while a third pair of nerves issues from the cord itself just anterior to the swelling, and is distributed along the anterior boundaries of the segment.

The œsophageal ganglia in the earthworm are, proportionately to the rest of the nervous system, much smaller than in the Nemertidæ; and this is perhaps due in great part to the existence of the numerous segmental ganglia in the former, which have no existence in the marine worms. The movements of the Nemertidæ, like those of the nematoids, are probably much more exclusively under the control of the œsophageal ganglia than are those of the segmented earthworm—in which each of the body-ganglia doubtless has much to do with bringing: about the contraction of contiguous muscles. The earthworm has also a more complex visceral structure than is to be met with among the Nemertidæ; and, moreover, it presents more distinct evidences of a nervous interconnection between the different organs of the body and some of the principal nerve-centres. Lockhart Clarke has described a complicated ganglionic network on each side of the œsophagus, starting from the commissures and sending prolongations to the intestine and other parts. By means of this principal visceral system of nerves, the internal organs are brought into relation with one another, and with the nervous system of animal life—that is, with those parts having to do more especially with the relation of the organism to its medium.

The upper or supra-œsophageal ganglia, representing the brain of the earthworm, receive a nerve-trunk on each side, composed of fibres coming from the tactile upper lip, and, as no sensory filaments of a different order are known to be immediately connected therewith, the functions of the brain in this animal must be comparatively simple. This upper lip contains a certain amount of diffused pigment, though there are no signs of the existence of distinct ocelli. I have spoken of the part as a special organ of touch, but it is equally probable that it may be capable of receiving more special impressions representing rudimentary tastes. The separation between these modes of sensibility may in such low organisms be somewhat indefinite.