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, witb 
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-n the 
latter. Thus the proper 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 first 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. 
