550 CRUSTACEA. 



to the mouth, and to maintain aerating currents in the water, 

 from whose peculiar character the name of the group is derived. 

 374. The chief points of interest to the Microscopist in the 

 more highly organized forms of Crustacea, are furnished by the 

 structure of the shell, and by the metamorphosis of the larvae, 

 both of which may be best studied in the commonest kinds. 

 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 sub- 

 stance. The innermost and even the middle layers, however, 

 maybe altogether wanting ; thus in the Phyllosomce or "glass- 

 crabs," the envelope is formed by 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 laorny 

 investment is made up of hexagonal thick-walled cells. (It may 

 be here noticed, that the carapace of Daphnia, Branchipus, and 

 some other Entomostraca, exhibits the hexagonal division ; whilst 

 in many species this is not distinguishable.) It is in the large 

 thick-shelled Crabs that we find the three layers most differen- 

 tiated. Thus in the common Cancer pagurm, 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 colored parts of the shell is contained, may be easily 

 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 magnify- 

 ing power of 250 diameters, driving a strong light through it 

 with the achromatic condenser; whilst the tubular structure ol 

 the thick inner layer may be readily demonstrated, by means of 

 sections parallel and perpendicular to its surface. This struc- 

 ture, which very strongly resembles that of dentine ( 406), 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 dif- 

 ference in the molecular arrangement of the mineral particles, 

 the organic structure being precisely the same) is much denser 

 than the rest of the shell, the former having almost the semi- 

 transparency 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 increased by the presence, at tolerably regular 

 intervals, of minute sinuosities corresponding with the lamina- 

 tions of the shell, which seem, like the " secondary curvatures" 

 of the dentinal tubuli, to indicate successive stages in the calcifi- 

 cation of the animal basis. This inner layer rises up through 

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

 tions ; and it is from the deficiency of the pigmentary layer at 

 these parts, that the colored portion of the shell derives its 



