DECAPOD CRUSTACEA I SHELL ; DEVELOPMENT. 649" 



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 re- 

 semblance is still further increased by the presence, at tolerably 

 regular intervals, of minute sinuosities corresponding with the 

 lamiuations of the shell, which seem, like the ' secondary curva- 

 tures ' of the Dentinal Tubuli, to indicate successive stages in the 

 calcification of the Animal basis. In thin sections of the Areo- 

 lated layer it may be seen that the apparent walls of the areola? 

 are merely translucent spaces from which the tubuli are absent, 

 their orifices being abundant in the intervening spaces.* The 

 tubular layer rises-up through the pigmentary layer of the Crab's 

 shell in little papillary elevations, which seem to be concretionary 

 nodules ; 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 Decapods ; thus in the 

 Prawns there are large stellate Pigment-spots (resembling those of 

 Frogs, Fig. 393, c), the colours of which are often in remarkable 

 conformity with those of the bottom of the Rock-pools frequented 

 by these creatures ; whilst in the Shrimps there is seldom any dis- 

 tinct 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 concretionary aggregation of the calcifying 

 deposit (§ 599). 



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

 difference 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 Metamorphosis, the young of the Land- 

 Crab and the Cray-fish 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 Metamorphosis, 

 whether they are afterwards to belong to the Macrourous (long- 

 tailed) or to the Brachyourous (short-tailed) division of the group ; 

 and the forms of these Larvae are so peculiar, and so entirely 

 different from any of those into which they are ultimately to be 

 developed, that they were considered as belonging to a distinct 



* The Author is now quite satisfied of the correctness of the interpre- 

 tation 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. William- 

 son (' 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 Associa- 

 tion " for 1847, p. 128; as indicating a Cellular structure in this layer. 



