278 PHYSIOLOGY OF THE INVERTEBRATA. 



such egg-shell formation and bone formation, as the carbonate 

 of lime is deposited in the chitinous portion of growing 

 epithelial cells in the crab shell." 



" In the secreting layer of the mantle of certain Molluscs 

 the lime in the epithelial cells is principally phosphate, 

 whilst the fluid bathing its outer surface and the shells them- 

 selves contain the lime, principally in the form of a carbonate. 

 If there is a definite interval between the secreting surface 

 and the area of deposition, or if much chitin or other tissue 

 is developed between the actively secreting cells and the 

 tissue in which the lime is deposited, there is always a greater 

 tendency to the formation and deposition of carbonate of 

 lime." 



Phosphates of the alkalies and alkaline earths occur in the 

 blood or nutritive fluid, and the latter acts as a carrier of 

 lime, &c., to every part of the body where carbonic anhydride 

 may be given off"; thus carbonate of lime is formed, and the 

 phosphoric acid re-enters the circulation. 



As already stated, the embryonic and young shells of the 

 ZamcUihranchiata are richer in phosphate of lime than the 

 shells of the fully-grown animal. No doubt as greater 

 activity goes on a larger amount of carbonic anhydride is 

 produced, and by this means more carbonate of lime is de- 

 posited than phosphate of lime. 



"W hen alkaline phosphates, associated with lime and albumin, 

 preponderate in the blood, the lime so separated is in the 

 form of phosphate, as in bone formation ; when these are 

 partially replaced by an excess of alkaline carbonates, as in 

 the majority of marine animals, the lime is secreted as car- 

 bonate. 



The corals have a secreting layer of cells which, according 

 to Irvine and Woodhead, produce chitin — chitin infiltrated 

 with calcium carbonate, and almost pure calcium carbonate, 

 with a small quantity of cementing organic material. 



The carbonate of lime is formed by the ammonium car- 

 bonate produced by the decomposition of the effete products 



