406 CBUSTACEA. 



perceive external impressions and originate motions, but 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 larvaB 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 Deca- 

 pods, 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 PJiyllo- 

 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 magni- 

 fying power of 250 diameters, driving a strong light through it ; whilst 

 the tubular structure of the thick inner layer may be readily demon- 

 strated 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 

 * Microscopic Journal. 



