PHYSIOLOGY OF THE INVERTEBRATA. 281 



" As the process of shell-formation must necessarily go on 

 slowly, it is not at all_astoniBhing that such a small propor- 

 tion of carbonic anhydride should be found in the mucous 

 material. It is used up as it is formed in laying down the 

 carbonate of lime of the shell. 



" As regards the proportion of the lime salts and chitin, 

 Schmidt found that the amount of earthy phosphate increases 

 in proportion to the quantity of chitinous tissue present in 

 the basement structure : 



" He argued from this that the calcium (lime) phosphate 

 is in intimate relation with cell-formation." Btit Irvine 

 and Woodhead think that as the chitin becomes older and 

 thicker the cellular layer becomes less active, less carbonate 

 is formed, and that there is thus a more direct passage out- 

 wards of the phosphate. In their papers already mentioned, 

 Irvine and Woodhead give the following analyses as showing 

 the comparative amount of calcareous and organic matter in 

 the common edible crab : 



Water, blood, salts, &c 6,646 grains 



Flesh (gave 14. 56 of ash containing 4.94 lime phosphate) . 295 „ 



Outer calcareous structure 2,956 „ 



Inner calcareous structure 103 n 



10,000 „ 



The calcareous structure consisted of : 



