PHYSIOLOGY OF THE INVERTEBRATA. 279 



of animals, as urea, &c., decomposing the calcium sulphate 

 in the sea water with the formation of calcium carbonate. 



In the blood of the lime-secreting Invertebrates there are 

 phosphates of lime and soda, along with alkaline chlorides, 

 carbonates, and sulphates associated with albuminous matter, 

 carbonic anhydride and oxygen being also present in varying 

 quantities. This blood is alkaline, which is due to the pre- 

 sence of alkaline phosphates and carbonates. 



Dr. Schmidt found that the bloo,d of Anodonta cygnm was 

 slightly alkaline ; and on evaporation it yielded crystals of 

 calcium carbonate resembling gaylussite. " These could not 

 have been present originally in the alkaline fluid, and it is 

 probable that they were produced by the formation of ammo- 

 nium carbonate from the decomposition of urea* and nitro- 

 genous organic matter." 



The membrane which secretes chitin also brings lime to 

 the surface, and in performing its protoplasmic function car- 

 bonic anhydride is set free ; this readily forms calcium car- 

 bonate after decomposing certain lime salts. " But it must 

 be noted that the chitin is directly in contact with the upper 

 secreting cells, in fact, the younger layers of chitin still form 

 the upper or old6r portion of the cell." Irvine and Wood- 

 head "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 into the carbonate. As the car- 

 bonate of lime is formed the free phosphoric acid is apparently 

 reabsorbed and utilised afresh. In proof of this fact, and as 

 bearing on the whole question of lime secretion, we refer to 

 the investigations of 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 albumin with phosphate of 



* Urea and uric acid are present in the excreta of Anodonta, see the 

 author's paper in the Chemical News, vol. 5I1 P- 241. 



