the Skeletons of Mollusca, Crustacea and Echinodermata. 387 



internally a thick calcareous layer, the sm-face of which is 

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

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

 structm-eless horny epidermis. The pigment-cells till 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 tlat upon the whole, adhering to the tops of 

 the papillse ; 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 tu^buli, which run nearly 

 parallel to one another from one sm-face 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 A^alentin, in his monograph of the 

 anatomy of that genus, published last year in the beautiful 

 work of ]\I. 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 structm-e, occur beneath the integument of some Holothu- 

 ridvB ; 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 gi-oup 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. 



