the Skeletons of Mollusca, Crustacea and Echinodermata. 887 



internally a thick calcareous layer, the surface of which is 

 raised up in little papillary elevations ; upon this lies a layer of 

 pigment-cells, to which the colour is due ; and externally is a 

 structureless horny epidermis. The pigment-cells fill up the 

 valleys or intervals between the papillary elevations of the calca- 

 reous layer, but do not cover the latter ; the epidermis, which is 

 quite smooth, lies flat upon the whole, adhering to the tops of 

 the papillae ; so that, when the shell is closely examined from the 

 outside, it is seen to have a minutely-speckled appearance, the 

 red ground given by the pigment-cells being studded with the 

 white tops of the papillse which rise up through it. 



The calcareous layer is composed of a substance exactly ana- 

 logous to ivory, being very transparent and apparently homo- 

 geneous, when cut into very thin slices, and being perforated by 

 an immense number of minute sinuous tubuli, which run nearly 

 parallel to one another from one surface of the shell to the 

 other. This arrangement may be seen by making a thin section 

 of any part of the shell ; but it may be shown particularly well 

 in the end of the claw, which is thicker and of denser texture 

 than the rest. A transverse section of this shows the tubes ra- 

 diating from the central cavity towards the external surface, and 

 would, I feel assured, be regarded by the most experienced ob- 

 server as the section of a tooth, if he were not informed of its 

 real nature. 



Echinodermata. 



The microscopic structure of the shell of Echinus has been 

 very completely analysed by Valentin, in his monograph of the 

 anatomy of that genus, published last year in the beautiful 

 work of M. Agassiz on this group. Before becoming acquainted 

 with his researches, I had arrived at conclusions exactly identi- 

 cal, and had extended my inquiries to all the other most import- 

 ant genera of the Echinida, Stellerida and Crinoidea. The result 

 has been extremely interesting. Every part of the skeleton in 

 these groups is made up of a structure so uniform, and at the 

 same time so unlike everything else, as to be most characteristic 

 of the group ; so that there could be no hesitation, wherever the 

 merest fragment of this structure might be distinguishable, in 

 pronouncing it to have belonged to an Echinoderm. I have 

 reason to believe that minute calcareous plates, having an iden- 

 tical structure, occur beneath the integument of some Holothu- 

 rid(2 ) if this be the case (which, for want of a specimen, I have 

 not yet had the opportunity of ascertaining), the universality of 

 the character, as distinguishing the whole group of Echinoder- 

 mata, will be completely established. 



The structure to which I allude consists of a series of very 

 thin plates, each perforated by a number of round apertures, 



