(1037). The medulla spinalis, which, as we shall see hereafter, corresponds to the ventral chain of ganglia in articulated animals, can perceive external impressions and originate motions, hut not feel pain; hence we may justly conclude that, in the Homogangliata likewise, the supra-cesophageal ganglia (the representatives of the brain, and the sole correspondents with the instruments of the higher senses) are alone capable of appreciating sensations of a painful character. Thus, then, we arrive at a very important conclusion, namely, that the perception of pain depends upon the development of the encephalic masses, and consequently that as this part of the nervous system becomes more perfect, the power of feeling painful impressions increases in the same ratio - or, in other words, that inasmuch as the strength, activity, and intelligence of an animal, by which it can escape from pain, depend upon the perfection of the brain, so does the perception of torture depend upon the condition of the same organ. How far the feeling of pain is acutely developed in the animals we are now considering is deducible from every-day observation.

The Fly seized by the leg will leave its limb behind, and alight with apparent unconcern to regale upon the nearest sweets within its reach; the Caterpillar enjoys,.to all appearance, a tranquil existence while the larvae of the Ichneumon, hatched in its body, devour its very viscera; and in the Crustacea before us, of so little importance is the loss of a leg, that the Lobster will throw off its claws if alarmed by the report of a cannon.

(1038). We learn from Dr. Williamson* that the shell of the Decapods, in its most complete form, consists of three strata: namely, 1. a horny structureless layer, covering the exterior; 2. a cellular stratum; and 3. a laminated tubular substance. The innermost, and even the middle layers, however, may be altogether wanting. Thus, in Phyllo-soma (Glass-crab) the envelope is formed of the transparent horny layer alone; and in many of the small Crabs belonging to the genus Portuna, the whole substance of the carapace beneath the horny investment is made up of hexagonal thick-walled cells. It is in the large thick-shelled Crabs that we find the three layers most differentiated. Thus, in the common Cancer pagurus we may easily separate the structureless horny covering after a short maceration in dilute acid; the cellular layer, in which the pigmentary matter of the coloured parts of the shell is contained, may then be brought into view by grinding away as flat a piece as can be selected from the inner side (having first cemented the outer surface to the glass slide), and by examining this with a magnifying power of 250 diameters, driving a strong light through it; whilst the tubular structure of the thick inner layer may be readily demonstrated by means of sections parallel and perpendicular to its surface.

This structure, which very strongly resembles dentine, save that the tubuli do not branch, but remain of the same size through their whole course, may be particularly well seen in the black extremity of the claw, which is much denser than the rest of the shell, the former having almost the semitransparency of ivory, whilst the latter has a chalky opacity. In a transverse section of the claw, the tubuli may he seen to radiate from the central cavity towards the surface, so as very strongly to resemhle their radiation in a tooth; and the resemblance is still further increased by the presence, at tolerably regular intervals, of minute sinuosities corresponding with the laminations of the shell, which seem, like the secondary curvatures of the dentinal tubuli, to indicate successive stages in the calcification of the animal basis. This inner layer rises up, through the pigmentary layer of the Crab's shell, in little papillary elevations; and it is from the deficiency of the pigmentary layer at these parts that the coloured portion of the shell derives its minutely speckled appearance.

Many departures from this type are presented by the different species of Crustacea. Thus, in the Prawns there are large stellate pigment-cells, the colours of which are often in remarkable conformity with those of the rock-pools frequented by these creatures; whilst in the Shrimps there is seldom any very distinct trace of the cellular layer, and the calcareous portion of the skeleton is disposed in the form of concentric rings, an approach to which arrangement is seen in the papillae of the surface of the deepest layer in the Crab's shell.

* Microscopic Journal.

(1039). The singular power of breaking off their own limbs, alluded to in paragraph 1037, is possessed by many Crustacea, and is a very indispensable provision in their economy. "We have already found the blood-vessels of these animals to be of a delicate structure; and the veins being wide sinuses whose walls possess little contractility, the fracture of a limb would inevitably produce an abundant and speedily fatal haemorrhage were there not some contrivance to remedy the otherwise unavoidable results of such a catastrophe. Should the claw of a Lobster, for example, be accidentally damaged by accidents to which creatures encased in such brittle armour must be perpetually exposed, the animal at once breaks off the injured member at a particular part, namely, at a point in the second piece from the body; and by this operation, which seems to produce no pain, the bleeding is effectually stanched*.

* Mr. Spence Bate gives the following account of this remarkable process: - "When a limb is injured, all Crustaceans have the power of rejecting it, except the wound be below the last joint. This is done by a violent muscular contraction, finishing with a blow from another limb, or against some foreign body. The amputation is the work of a few seconds, except when they have but recently cast their exuviae; at such times the wounded limb will sometimes remain for half an hour or longer before it is rejected.