Page:Popular Science Monthly Volume 9.djvu/737

Rh wood-lice (such as talitrus and oniscus), in which the body is elongated and composed of many almost similar segments, the nervous system is not very different from that of the more highly-organized worms.

In slightly higher forms of Crustacea, however, the two divisions of the originally double ventral cord approximate and become fused together, while, at the same time, the equality of its ganglia diminishes. Thus, in such forms as the lobster and the crayfish, the ganglia of the thorax which supply nerves to the limbs are distinctly larger than those of the abdominal segments, though these are also of good size, since the tail-segments are actively called into play during locomotion.

In the prawn a further development and concentration of the nervous system is seen. The thoracic ganglia are fused into a single elliptical mass, though those of the abdominal segments still remain separate.

But in the ordinary edible crab and its allies (Fig. 4), a still more remarkable concentration of the nervous system is met with. All the thoracic and all the abdominal ganglia are here fused into one large perforated mass of nervous matter (c), situated near the middle of the ventral region of the body. From this large compound ganglionic mass nerves are given off to the limbs, to the abortive tail, and to other parts.

The brain of the crab (a) is represented by a rather small bilobed ganglion. It receives nerves from the pedunculated compound eyes, from the two pairs of antennæ, and from the palpi-bearing mandibles. The posterior antennæ (or antennules, as they are sometimes termed) contain in their basal joint a body which is supposed to represent an olfactory organ, though others have regarded it (on very insufficient grounds) as an organ of hearing. The rather small bilobed brain is, indeed, regarded by many naturalists as essentially composed of three pairs of ganglia, completely fused into one another, but in relation with the three pairs of sensory organs—the eyes, the tactile antennæ, and the supposed olfactory antennules. It is connected, by means of a long cord (b, b), on each side of the œsophagus, with the anterior extremity of the great ventral ganglion. These cords are long because of the absence of any separate sub-œsophageal ganglia, and because of the comparative distance of the great ventral nervous mass into the composition of which these ganglia enter. The great length of the œsophageal cords is one of the most notable characteristics of the nervous system of the higher Crustacea.

The "stomato-gastric" system of Crustacea is closely similar to that which exists in centipedes. One part of it is given off from the œsophageal cord on each side, while another median branch proceeds from the posterior part of the united cephalic ganglia, as in Iulus (Fig. 3,f).