Page:Popular Science Monthly Volume 7.djvu/60

50 The general plan of the nervous organization throughout this large division is that just described, and commencing with cephalous animals. It consists of symmetrically-arranged ganglia—a sensorium, and the necessary communicating nerves.

And what are the actions by which this improved nervous system, characterizing all this large class of animals, manifests itself?

Taking the two extremes in the class of well-characterized articulates, the centipede at the lower, and wasps and bees at the upper, we notice a wide difference in the complexity of the general organization and a difference equally marked in the character of the corresponding actions. In the centipede, the general structure consists of a head furnished with certain organs of special sense, and a body made up of a series of segments—each, with the exception of the last, being a repetition of the others, and each being furnished with a single pair of legs.



To preside over these organs of locomotion, the nervous system is distributed as follows: Each segment is supplied with a double ganglion, or nerve-centre, all being arranged in regular order just beneath the alimentary canal, and in the head is placed the sensorium; all the different ganglia are connected by a fine double-nerve filament called the ventral cord, and through this means also all are in communication with the sensorium. So each ganglion is in communication with its own particular pair of locomotive organs, with each of the other ganglia, and with the sensorium.

The chief action of this animal consists in the movement of these organs of locomotion; and this it does in response to direct irritation from without, or in consequence or sensations received at the sensorium through the organs of special sense, and thence transmitted to the several ganglia. So long, then, as this nervous communication remains unbroken, the action of all the segments composing the body is harmonious, and is performed with a direct relation to the guiding influence of the sensorium. This is the normal action of the animal. But suppose, now, that the head, with the sensorium, be removed while the creature is in motion, the legs still perform their office, carrying the body forward in its accustomed manner until it meets an obstruction, when its progress is stopped, though the legs still continue to move. If the body be divided, similar results follow: the forward part moves on under the guidance of the sensorium, avoiding or overcoming obstacles according as sight or other sensations