2IO BULLETIN OP THE BUREAU OF FISHERIES. 



The composition of the gastroliths is similar to that of the shell, a conclusion which 

 we should be led to draw from the fact that these bodies are specialized parts of the dead 

 chitinous integument. The same substances are found in both, but in different propor- 

 tions. The gastroliths are far richer in Ume, chiefly in the form of carbonate (CaCOj), 

 than is the shell, and the amounts of magnesium carbonate (MgCOj), alumina (AI2O3), 

 ferric oxide (Fe203) , and silica (SiOj) are more or less reduced. 



Lime estimated as carbonate (CaCOj) constitutes about three-fourths of the gastro- 

 lith, but less than two-fifths of the carapace. Lime reckoned as phosphate (Ca3(P04)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 various salts and oxides. 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. 



Since the total quantity of lime contained in the gastroliths is but a small fraction 

 of 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. Lobsters when young and sometimes when adult not only eat their 

 own cast after molting, but swallow fragments of shells and other calcareous materials, 

 which are dissolved in the stomach and help to strengthen the new shell. 



Williams (279), who has recently studied this subject, has added some important 

 facts to our knowledge of the gastroliths. He found that while absent in the larvae 

 they made their appearance at the fourth stage, when the shell begins to receive deposits 

 of lime, and at about the middle of this period. After the next molt the gastroliths 

 were dissolved in the course of a few hours, either remaining in place or falUng to the 

 bottom of the stomach sac, to be later broken up. With their dissolution there was 

 observed a gradual hardening of the gastric teeth, mandibles, and later of the chehpeds 

 and other parts. 



As soon as the gastroliths are dissolved [says Williams] , the lobster attacks his cast, beginning to eat 

 the bristles and small parts and proceeding to devour more or less of the harder parts. The newly 

 molted lobsters seldom seriously attack their sloughs within three or four hours, and generally eat the 

 greater part of the cast within twelve or eighteen hoturs. 



He therefore supports the older view that the gastroliths represent a store of lime 

 and other minerals reserved from the old shell for the immediate hardening of the new, 

 with the additional statement that this reserve is destined for particular parts — gastric 

 teeth, mandibles, and chelipeds — so that the cast and other calcareous matter within 

 easy reach may be quickly available. 



Stebbing (260) , who also has criticised the view that the gastroliths are primarily 

 excreted products, does not believe that such nicely adjusted structures can serve as 

 "mere off scourings of the body." 



The difficulties in the way of supposing that these interesting bodies are necessary 

 rather than incidental sources of lime to the newly molted lobster are by no means 

 removed by the observations quoted above. To be of service at all the carbonates of 



