348 ■ NOTES. 



each side of the anterior part of the cardiac division of the stomach. The 

 proper wall of the stomach is continued over the outer surface of the 

 prominence ; and, in fact, forms the outer wall of the chamber in which 

 the gastrolith is contained, the inner wall being formed by the cuti- 

 cular lining of the stomach. When the outer wall is cut through, it is 

 readily detached from the convex outer surface of the gastrolith, with 

 which it is in close contact. The inner surface of the gastrolith is usually 

 flat or slightly concave. Sometimes it is strongly adherent to the chi- 

 tonous cuticula ; but when fully formed it is readily detached fro_a the 

 latter. Thus the proijer wall of the stomach invests only the outer face 

 of the gastrolith, the inner face of which is adherent to, or at any rate in 

 close contact with, the cuticula. The gastrolith is by no means a mere 

 concretion, but is a cuticular growth, having a definite structure. Its 

 inner surface is smooth, but the outer surface is rough, from the projec- 

 tion of irregular ridges which form a kind of meshwork. A vertical sec- 

 tion shows that it is composed of thin superimposed layers, of which the 

 inner are parallel with the flat inner surface, while the outer becomes 

 gradually concentric with the outer surface. Moreover, the inner layers 

 are less calcified than the outer, the projections of the outer surface being 

 particularly dense and hard. In fact, the gastroliths are very similar to 

 other hard parts of the exoskeleton in structure, except that the densest 

 layers are nearest the epithelial substratum, instead of furthest away 

 from it. 



When ecdysis occurs, the gastroliths are cast off along with the gas- 

 tric armature in general, into the cavity of the stomach, and are there 

 dissolved, a new cuticle being formed external to them from the proper 

 wall of the stomach. The dissolved calcareous matter is probably used 

 up in the formation of the new exoskeleton. 



According to the observations of M. Chantran (Comptes Rendus, 

 LXXVIII. 1874) the gastroliths begin to be formed about forty days 

 before ecdysis takes place in crayfish of four years' old ; but the 

 interval is less in younger crayfish, and is not more than ten days 

 during the fii'st year after birth. When shed into the stomach during 

 ecdysis they are ground down, not merely dissolved. The process 

 of destruction and absorption takes twenty-four to thirty hours 

 in very young crayfish, seventy to eighty hours in adults. Unless 

 the gastroliths are normally developed and re-absorbed, ecdysis is 

 not healthily effected, and the crayfish dies in the course of the 

 process. 



