340 
Proceedings of Royal Society of Edinburgh. [sess. 
addition of carbonic acid, but this is not borne out, for it must be 
remembered that the acids cannot be free in the blood, as that fluid 
always gives an alkaline reaction, as both “ acid ” phosphates and 
carbonates are slightly alkaline. It is only when we have an evolu- 
tion of the free carbonic acid that we have any rearrangement of 
the alkalies and alkaline earths, and a deposition in an insoluble 
form of the phosphate of lime. 
In the case of the shell of the crab we have an intermediate con- 
dition between the shell of a hen’s egg and bone ; we have two 
processes going on, but in a somewhat different manner. It will 
be evident, from a careful consideration of the structure of the 
membrane that secretes the chitin, that it may do something more 
than attend to the formation of that substance, for it is found to 
contain a very considerable proportion of lime and a quantity of 
phosphoric and carbonic acids. We may, in fact, assign to it 
the role that we assigned to the epithelial layer of the oviduct ; it 
brings lime to the surface, and in performing its protoplasmic 
function carbonic acid is set free in a nascent condition, and in 
direct contact with the other lime salts ; we have, as a result, a 
large proportion of the lime passed in as carbonate. But it must 
be noted that the chitin is directly in contact with these upper 
secreting cells, in fact, the younger layers of chitin still form the 
upper or older portion of the cell. Here we maintain that the 
direct contact allows of the dialysis into the chitin of a portion of 
the phosphate of lime before it is completely transformed to the 
carbonate. 
As the carbonate of lime is formed the free phosphoric acid is 
apparently reabsorbed and utilised afresh. In proof of this latter 
fact, and as bearing on the whole question of lime secretion, we 
may be allowed to quote Schmidt,* who in speaking of Unio, 
Anodonta, and Helix describes the structure of the secreting 
membrane of the mantle as a layer of hexagonal cells on which is a 
structureless transparent membrane in which the lime is deposited, 
and ascribes to it the function of decomposing the blood, of secreting 
a compound of albumen with phosphate of lime next the shell, 
which is decomposable even by the carbonic acid of the air or of 
the water, but of retaining the phosphoric acid and returning 
* Loc. cit., p. 28. 
