lens appeared variegated by reddish sinuous markings, resembling blood vessels as they are seen in injected glands[20].

II. Tunics.—The coats that inclose the various branches of the nervous system in insects seem analogous to those of vertebrate animals. The first thing that strikes the eye, when these parts in a recent subject are submitted to a microscope, is a tissue of very delicate vessels, which ramify beyond the reach of the assisted sight; these are merely air-vessels or bronchiæ derived originally from the tracheæ of the animal: but besides these is an exterior and an interior tunic; the first corresponding with the dura mater of anatomists; and the other, which is the most delicate and incloses the cortical and medullary parts, with the pia mater[21].

III. Parts.—The nervous system of insects consists of the brain; the spinal marrow and its ganglions; and the nerves.

i. Brain.[22] Linné denied the existence of a brain in insects, and most modern physiologists seem to be of the same opinion. A part however, analogous to this important organ—at least in its situation, and in its emission of nerves to the principal organs of the senses, in which respect it certainly differs very materially from the upper cervical ganglion, which Dr. Virey regards as its analogue[23]—is certainly to be found in them; and as Messrs. Cuvier and Lamarck distinguish this part by the name of brain, we may continue to call it by that name without impropriety. The brain of insects, then, is distinguished from the succeeding ganglions of the spinal chord by its situation in the head, the middle of the internal cavity of which it occupies, and by being the only ganglion above the œsophagus. It is usually small, though in some cases larger than they are[24]. It consists of two lobes, more or less distinct and generally of a spherical form. In Oryctes nasicornis and Pontia Brassicæ the lobes are separated both before and behind[25]; while in the larva of Dytiscus marginalis, but not in the imago, in which there are two large hemispheres separated by a furrow, the brain is undivided[26]. Cuvier mentions the larva of a saw-fly in which this part is formed of four nearly equal spherical bulbs[27]: in the Scorpion (to judge by the figure of Treviranus[28]) the two lobes represent an equilateral triangle, the exterior angle of which terminates in several lesser spherical bulbs; in Acrida viridissima, Nepa cinerea, Clubiona atrox, and the common Louse, the lobes are pear-shaped[29].

ii. The spinal marrow and its ganglions[30]. From the posterior part of the brain of insects, but in the ground and water beetles (Eutrechina and Eunechina) from its sides below[31], issue two chords which diverging embrace the œsophagus, and dipping below it and the intestines,—a situation they maintain to the end of their course,—and in their further progress uniting at intervals and dilating into several knots or ganglions, compose their spinal marrow. This part is so named, from a supposed analogy to the spinal marrow of vertebrate animals, which however admits of some degree of doubt; yet, since it mixes the functions of that organ with those of the great sympathetic nerves, the denomination is not wholly improper, and may be retained. Though this chord is usually double when it first proceeds from the brain, and surrounds the œsophagus like a collar, yet in some insects it may be called a single chord. This is the case with that of the common louse, in which Swammerdam could perceive no opening for the transmission of the part just named[32]; if he was not mistaken in this, the brain, as well as the rest of the spinal marrow in that animal, would be below the intestines; from the figures of Treviranus it should seem that the spiders, at least Clubiona atrox, are similarly circumstanced[33]; in the cheese-maggot, which turns to a two-winged fly (Tyrophaga Casei), the chord is also single, but it has a small orifice through which the gullet passes[34]. At the union of the chords in other cases below that organ, a knot or ganglion is usually formed, and an alternate succession of internodes and ganglions commonly follows to the end. The internodes also may generally be stated to consist of a double chord, though in many cases the two chords unite and become one, or are distinguished only by a longitudinal furrow, and even where they are really distinct and separable, in the body of the insect they lie close together[35]. In the rhinoceros beetle (Oryctes nasicornis) and Acrida viridissima &c. all the internodes consist of a double chord