Page:Popular Science Monthly Volume 7.djvu/58

48 continuous lines, and called nerve-trunks or simply nerves. The ganglia are the centres of power. The trunks are the lines of communication between the ganglia and the different parts of the organism.

Without descending into the debatable ground between the vegetable and animal world (for, strange to say, the boundary-line between the modes of life apparently so different has never yet been established), let us examine an organism as lowly as any possessing a distinct and single nervous system—an ascidian mollusk—which simply means a bag-shaped, soft-bodied animal. It has no head, nor any



organs of sight, hearing, or smell. It consists of a sac, from the lower portion of which proceeds a stomach or digestive tube; these are covered with a muscular envelope, and the whole is inclosed in another envelope or membranous sac called a mantle. These coverings are pierced by two openings—a mouth for admitting water and the nutrient particles which it happens to contain into the inner sac or common reservoir, which also serves as a respiratory organ—and a vent, communicating directly with this sac, and also with the more circuitous digestive tube. A constant stream of water passes through the mouth into this common or respiratory sac, where, after having served its purpose of supplying oxygen to the system, a part is ejected directly through the vent, and the remainder, together with the nutrient particles, passes into the digestive tube, and thus finds its exit.

The nervous system, which is the part most important to our present purpose, is of the most simple kind; it consists of a single ganglion, situated between the two orifices of the body, with each of which it has lines of nerve-communication, and also with the various envelopes which mainly constitute the animal.

All the creature's movements must be carried on by means of this simple nervous arrangement; and, as it is fixed to one spot during its whole life, they must necessarily be of a very limited character; it has, in fact, but one movement—it contracts when touched. Suppose, for instance, some offending substance to have found its way into the common sac, the irritation caused by it is transferred along the