NATURAL HISTORY OP AMERICAN LOBSTER. 209 



If the shell of the lobster which is nearly ready to molt is removed, there will be seen 

 two glistening snow-white masses, one on either side of the stomach, A gastrolith 

 taken from a lobster 1 1 inches in length was an inch long, three-quarters of an inch wide, 

 and a quarter of an inch thick. Its outer convex side was applied to the sac in which it 

 lay, while its concave side was separated from the cavity of the stomach by the old cutic- 

 ular lining of this organ. When the stomach is raised the gastroliths almost break 

 through its delicate outer wall by their own weight. They lie between the old cuticular 

 lining of the stomach, which may be stripped off, and its delicate outer wall, next to 

 the body cavity. The impression of the gastrolithic plate (pi. xxxiii) is seen on the 

 new cuticular lining only. If the sacs in which they are formed are cut open, each 

 mass separates into hundreds of small ossicles or columns, the majority of which are 

 slender truncated prisms of irregular shapes and about one-fifth of an inch long. Each 

 ossicle resembles a piece of milk-white glass, with transparent edges, and is faintly 

 marked with transverse and longitudinal striations, like those seen in the cuticle. 



The gastroliths, though a part of the cuticle, are not regularly cast off during the molt, 

 but are retained in the stomach; when the old lining of this organ is withdrawn, they 

 are soon set free, and breaking up into their constituent parts are speedily dissolved. 

 Consequently it has been supposed that they served the function of providing a supply 

 of lime for hardening the new shell. Messrs. Irvine and Woodward (165), however, 

 have proved that the amount of calcareous matter obtained in this way is only about 

 one one-hundred-and-eighty-sixth part of that of the entire skeleton, and therefore too 

 insignificant to be of any practical value. Lime, moreover, is at hand in abundance 

 in the form of the shells and skeletal fragments of mollusks and other animals, which 

 lobsters make free use of at the time of the molt. 



We have suggested that the gastrolithic plates or sacs in the walls of the stomach 

 are organs for the excretion of lime, and that the gastroliths represent the hme removed 

 from the absorption areas previous to the molt. Upon this theory their retention and 

 absorption is an incident of no special importance (see 149, p. 93). 



The gastrolith of one of the common crayfishes (Cambarus robustus) when 4 inches 

 long is about the size of a split pea, 7 millimeters in diameter by 5 millimeters thick. 

 It shows no divisions into ossicles, but is a hard mass. The convex face is dull white 

 and nearly smooth, while the flattened side presents a brown circular scar with a white 

 center. In form and appearance it suggests a small mushroom with the stem cut off 

 close to the cap. In sectional view it shows concentric striations. 



Chemical analysis " has proved that lime salts as 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 

 proportion of organic matter present is considerably less than under other conditions. 

 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. Small quantities 

 of alumina and silica are normally present in both the shell and gastroHths. 



a See article by Prof. A. W. Smith, 252 of bibliography. 



