94 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



CHEMICAL ANALYSIS OF THE SHELL AND GASTROLITHS. 



It seemed to me that a chemical analysis of the shell of the lobster in its differ- 

 ent conditions imposed by the molting habit might prove of interest, especially when 

 compared with the composition of the gastroliths, and I am fortunate in being able to 

 add as an appendix of this work the results of several analyses made by my friend, 

 Professor Albert W. Smith. 



The most striking facts brought out by Professor Smith's work are, first, that lime 

 salts, carbonates and phosphates, form about half the constituents of the hard shell, 

 there being from three to five times as much carbonate as phosphate. We also find 

 that in the cast shell of the lobster, the brittleness of which we have already noticed, 

 the proportion of organic matter present is considerably less than under other condi- 

 tions. An absorption of organic matter thus takes place during the period in which 

 the new shell is formed, and this fact explains the fragility of the cast-off shell. 



It is also interesting to notice that small quantities of alumina and silica are 

 normally present in both the shell and gastrolith. 



The composition of the gastroliths is very like that of the shell, a conclusion 

 which we would be led to draw from the fact that the gastrolith is but a specialized 

 part of the dead chitinous integument. The same substances are found in both, but 

 in different proportions. The gastroliths are far richer in lime, chiefly in the form of 

 carbonate (0a0O 3 ), than is the shell, and the amounts of magnesium carbonate 

 (MgOO^), alumina ( A1 2 :! ), ferric oxide (Pe20 3 ), and silica (SiO>) are more or less reduced. 



Lime estimated as carbonate (0aCO 3 ) constitutes about three-fourths of the 

 gastrolith, but less than two-fifths of the carapace. Lime reckoned as phosphate 

 (0a 3 (PO 4 ) 2 ) forms about 10 per cent of the gastrolith and but little less in the case of 

 the shell; about 10 per cent of the gastrolith is water and organic matter, probably 

 mainly chitin, and the rest is made up of the various salts and oxides given in the table. 

 In the only molted shell analyzed about 38 per cent was water and organic matter, while 

 in two hard-shell lobsters this percentage was considerably greater, 42.21 in one case 

 and 51.80 in the other. 



The gastroliths of the crayfish were analyzed by Dulk (54) in 1834, 1 but apart from 

 this rough determination no later work has been done on this subject. 



He also analyzed the contents of the stomach of a crab newly molted, and found 

 a free volatile acid, probably hydrochloric, present, besides lime salts (53). 



THE HARDENING OF THE NEW SHELL. 



Since the total quautity of lime contained in the gastroliths is insignificant com- 

 pared with the amount necessary for building up the hard crust, the rapidity with 

 which the new shell hardens depends, in some measure, upon the individual, and 

 particularly upon the quality of its food. We have seen that the adolescent lobster, 

 under 4 inches long, after molting swallows fragments of shells and other calcareous 

 materials, which are dissolved in the stomach and help in strengthening the new shell. 

 It is possible that older lobsters have the same habit. 



1 The results of Dulk's work were as follows : 



Animal matter soluble in water 11. 43 "| 



Animal matter insoluble in water (probably chitin — Huxley) 4. 33 | 



Phosphate of lime 18. 60 ^98.93 



Carbonate of lime 63. 16 j 



Soda reckoned as carbonate , 1. 41 J 



