The chief part of the _nervous system_ is the ventral nerve-chain, which runs along the under-side of the body. This is a long cord having at intervals a series of knots or swellings, the ganglia or nerve-centres, from which nerves are given off to the appendages and to the organs of the body. In the hinder part of the thorax and in the abdomen there is a ganglion in each somite, but in front these ganglia become crowded together and coalesced, so that we find only a single large ganglion, corresponding to the somites from that of the mandibles to that of the third maxillipeds. Between the ganglia the cord is really double, although for the greater part of its length the two parts are more or less completely fused into one. In front of the head and above the gullet is a ganglion which sends nerves to the eyes, antennules and antennæ, and is known as the _brain_, although it is, perhaps, hardly so important as that name would suggest. It is connected with the ventral chain by two cords that pass on either side of the gullet. The _eyes_, as already mentioned, are set on movable stalks, so that they can be turned in any direction at the will of the animal, and are of the type known as "compound eyes." If the convex black area at the end of the eye-stalk be examined with a strong lens, it will be seen that the membrane which covers it is divided up into a beautifully regular series of square facets. This membrane is a thin and transparent continuation of the chitinous covering of the body, and if it be stripped off and examined under a microscope, it will be found that each facet is capable of acting as a lens and forming an image of external objects. It is not to be supposed, however, that the Lobster sees a separate image in each of the facets, some thirteen thousand in number, which go to make up each eye. In the interior of the eye, at some distance from the surface, are a large number of rod-like bodies, connected with the fibres of the optic nerve, and believed to be the actual organs for the perception of light. Each rod corresponds to one of the facets, and as it lies at the bottom of a long conical tube, of which the walls are covered with dark pigment, it can only receive light from a single point in line with the axis of the tube. In this way the image of any object will be built up, like a mosaic, out of the impressions of light and darkness received through the separate facets, and transmitted to the underlying rods. It has been shown in some Crustacea that, when the animal is in a very dim light, the curtain of pigment separating the tubes is partially withdrawn, so that the light from each facet can reach, not one, but several rods. In this way the images of objects received are much brighter, although they are less sharply defined.