Page:Popular Science Monthly Volume 7.djvu/59

Rh communicating nerves to the ganglion; there a "motor impulse" is generated, which again is transferred along the communicating lines from the ganglion to the muscular envelope, causing it to contract and eject the water which it contained, together with the offending substance. This is all the action the creature is capable of, and this it repeats whenever and wherever an irritation is applied. It is the simplest action, so far as we know, requiring a nervous system for its accomplishment, and has received the name of reflex action.

Without following each shade of improvement in the nervous organization of the mollusks, or soft-bodied animals, it may suffice to say that the same general arrangement holds throughout all the lower members of the series, new ganglia being added to meet the needs of a more complex organization until, in the highest members, as, for instance, in snails and the cuttle-fish, important changes are found to have occurred. Instead of the headless and irregular masses which have constituted the bodies up to this point, we now find an animal comparatively symmetrical in form, with a distinct head, and imperfectly-developed organs of special sense. Here, then, occurs the division between the two great classes of animals known as cephalous and acephalous; all the lower species of mollusks, the ascidians, mussels, oysters, and the like, belong to the acephalous or headless class, while with the highest species, snails, the nautilus, and the cuttle-fish, commences the other great class known as cephalous animals, or those having distinct heads.

The advance in the nervous system is correspondingly great; instead of the irregularly-situated ganglia hitherto met with, we find them arranged in pairs, to serve the purposes of the more symmetrical body; but a still more marked and important advance is found in the fact that each organ of special sense (sight, hearing, etc.) is furnished with a separate nerve-centre or ganglion; all of which, being brought together in the head of the animal, constitute what is known as the cephalic ganglia, or sensorium, an incipient brain.

The function of this nervous system is not merely to respond to irritation applied directly to the body, but also to respond to certain stimuli, such as that afforded by sight and hearing, received at the sensorium through the organs of special sense, and thence transferred to the different ganglia. This new stimulus is called sensation, and it is one to which the ganglionic system responds with almost the same alacrity that it does to direct contact.

Leaving now the grand division of mollusks, we ascend to another division in the animal world, namely, the Articulates. It embraces such marine animals as the crab, lobster, and crayfish; and on the land, worms, centipedes, and all the numerous tribes of insects. The characteristic of the whole division is, that the body is made up of rings or segments, joined and moving one upon the other, and hence the name articulates.