28 SCHMIDT ON THE COMPARATIVE PHYSIOLOGY 



stages ; and in these the embryo of the lobster must be of suffi- 

 cient size to allow of our tracing the transition stages by ele- 

 mentary analysis. 



2. By the accurate study of the relations accompanying the 

 annual formation of the skin, likewise in those species which can 

 be obtained of the largest size, and in quantity, where the em- 

 bryonic process of formation of the tegument must be repeated, 

 at least in its essential features. I must leave this for future 

 accomplishment ; it requires long residence at the sea-side, which 

 I have been unable to obtain during the course of the last summer. 



2. Mollusca. 



In the general part I stated that the cutaneous system of these 

 animals was purely animal. This position is based upon the 

 following observations : the shells of Unio and Anodonta consist 

 of superimposed layers of calcareous salts (carbonate of lime) and 

 albuminates. The latter are brought into view by the action of 

 acid solvents ; they then remain behind as white structureless 

 lamellae. The lime probably exists in the shell in the form of 

 acute rhombohedra, which are arranged in rows* ; at least, when 

 treated with acetic acid before undergoing solution, it is resolved 

 into fibres, in which I thought I could distinguish the separate 

 elements composing it. The iridescence of the shells, a phagno- 

 menon resulting from interference, is effected by the delicate inter- 

 stices of these fibres. These calcareous shells are a product of se- 

 cretion from the mantle ; they are externally covered by a mem- 

 brane resembling horn, which thickens into a Hgament at the 

 hinge : this, in minute structure and chemical properties, exhibits 

 the reactions of a duplicature of the mantle. Thus, its external 

 layer consists of an epithelium composed of five or six-sided cells, 

 containing nuclei and filled with a bluish-green or brown pigment, 

 and between which we find one or more layers of fibres resem- 

 bling those of cellular tissue. It is impossible to free it com- 

 pletely from the finely-divided silicates which adhere to it, and 

 the presence of which does not at aU interfere with the determi- 

 nation of the amount of nitrogen. 



0*2 13 of this duplicature of the mantle removed by the forceps 

 (and dried at 248" F.) gave 0-037 ash =17'4 per cent. 



0*369 gave 0*739 ammonio-chloride of platinum =15'22 per 



* For a description of the beauty of those crystals in Teredo gigantea, see 

 Home, Philosophical Transactions, 1806, p. 27(i. 



