CRUSTACEA. 215 



Thus, in the common Cancer pagurvs, we may easily separate the struc- 

 tureless horny covering after a short maceration in dilute acid; the 

 areolated layer, in which the pigmentary matter of the colored parts of 

 the shell is chiefly contained, may be easily brought into view by grind- 

 ing away from the inner side as flat a piece as can be selected, 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 with the Achromatic Condenser; 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 resem- 

 bles that of dentine ( 655), 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 (apparently from 

 some peculiarity in the molecular arrangement of its mineral particles) is 

 much denser than the rest of the shell; the former having almost the 

 semi-transparence of ivory, whilst the latter has a chalky opacity. In 

 a transverse section of the claw, the tubuli may be seen to radiate from 

 the central cavity towards the surface, so as very strongly to resemble 

 their arrangement in a tooth; and the resemblance is still further in- 

 creased by the presence, at tolerably regular intervals, of minute sinuosi- 

 ties 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. In thin sections of the 

 areolated layer it may be seen that the apparent walls of the areolas are 

 merely translucent spaces from which the tubuli are absent, their orifices 

 being abundant in the intervening spaces. 1 The tubular layer rises up 

 through the pigmentary layer of the Crab's shell in little papillary eleva- 

 tions, which seem to be concretionary nodules; and it is from the deficiency 

 of the pigmentary layer at these parts, that the colored portion of the 

 shell derives its minutely-speckled appearance. Many departures from 

 this type are presented by the different species of Decapods; thus, in the 

 Prawns, there are large stellate pigment-spots (resembling those of 

 Frogs, Fig. 465, c), the colors of which are often in remarkable con- 

 formity with those of the bottom of the rock-pools frequented by these 

 creatures; whilst in the Shrimps there is seldom any distinct trace of 

 the areolated layer, and the calcareous portion of the skeleton is disposed 

 in the form of concentric rings, which seem to be the result of the con- 

 cretionary aggregation of the calcifying deposit ( 713). 



614. It is a very curious circumstance, that a strongly-marked differ- 

 ence exists between Crustaceans that are otherwise very closely allied, in 

 regard to the degree of change to which their young are subject in their 

 progress towards the adult condition. For whilst the common Crab, 

 Lobster, Spiny Lobster, Prawn, and Shrimp undergo a regular meta- 

 morphosis, the young of the Cray -fish and some Land-crabs come forth 

 from the egg in a form which corresponds in all essential particulars 

 with that of their parents. Generally speaking, a strong resemblance 

 exists among the young of all the species of Decapods which undergo a 



1 The Author is now quite satisfied of the correctness of the interpretation 

 put by Prof. Huxley (see his Article, ' Tegumentary Organs,' in the " Cyclop, of 

 Anat. and Phys.," Vol. v., p. 487) and by Prof. W. C. Williamson (' On some 

 Histological Features in the Shells of Crustacea,' in " Quart. Journ. of Microsc. 

 Science," Vol. viii., 1860, p. 38), upon the appearances which he formerly 

 described ("Reports of British Association" for 1847, p. 128) as indicating a cellu- 

 lar structure in this layer. 



