172 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



THE GASTROLITH. 



(Plate V, Fig. 5, and Plate YI, Figs. 7 and 8.) 



The gastrolith is an irregular triangularly-ovate plate of lime, 

 formed of numerous angular prisms that extend from the epithelium 

 of the stomach wall to a plate of chitin which lines the stomach and 

 which is secreted early in the period between two molts by the epithe- 

 lium which later produces the gastrolith. Each prism appears first 

 as a disc in the center of an irregular shallow pit in the secretory 

 epithelium and gradually is pushed inward until it becomes a slender 

 irregular prism whose inner and outer ends are rounded. These 

 prisms increase in size until they fit snugly together and their rounded 

 ends give the surfaces of the gastrolith a knobbed appearance. The 

 gastrolith becomes quite thick before the molt (one-quarter of an 

 inch in a lobster 11 inches long [Herrick], .5 mm. in a fourth-stage 

 lobster). After the molt the gastrolith falls into the lumen of the 

 stomach, is broken up, and dissolved in a few hours. 



Two rival theories of the function of the gastrolith are widely 

 known, and we will present a third theory which is a modification of 

 the older of the two theories in accordance with some newly dis- 

 covered facts. The older theory regards the gastroliths as a mass of 

 mineral matter reserved when the old shell is cast for the hardening 

 of the new shell. The objection offered to this theory is that the 

 gastroliths are entirely too small to be effective for this purpose.. 

 Herrick suggests that the gastroliths are formed by the deposition 

 of the lime absorbed from the shell in preparation for the molt and 

 that they have no further important function than to remove a 

 temporary excess of lime from the blood. 



Thanks to the courtesy of the Commission of Inland Fisheries, we 

 had at our disposal last summer a large number of larval lobsters of 

 known age. 



We found that the gastroliths do not appear in the first three 

 stages, nor would we expect mineral matter in the wall of the stomach 



