that they contain carbonate of lime. If the cells are immersed in a very
little acid, they become more transparent, look like ghosts, and are soon
lost to view; but if much acid is added, they disappear instantly. After
a very large number have been dissolved, a flocculent residue is left,
which apparently consists of the delicate ruptured cell-walls. In the
two posterior pairs of glands the carbonate of lime contained in the
cells occasionally aggregates into small rhombic crystals or into
concretions, which lie between the lamell ; but I have seen only one
case, and Claparede only a very few such cases.
The two anterior glands differ a little in shape from the four posterior
ones, by being more oval. They differ also conspicuously in generally
containing several small, or two or three larger, or a single very large
concretion of carbonate of lime, as much as 11 2 mm. in diameter. When a
gland includes only a few very small concretions, or, as sometimes
happens, none at all, it is easily overlooked. The large concretions are
round or oval, and exteriorly almost smooth. One was found which filled
up not only the whole gland, as is often the case, but its neck; so that
it resembled an olive-oil flask in shape. These concretions when broken
are seen to be more or less crystalline in structure. How they escape
from the gland is a marvel; but that they do escape is certain, for they
are often found in the gizzard, intestines, and in the castings of worms,
both with those kept in confinement and those in a state of nature.
Claparede says very little about the structure of the two anterior
glands, and he supposes that the calcareous matter of which the
concretions are formed is derived from the four posterior glands. But if
an anterior gland which contains only small concretions is placed in
acetic acid and afterwards dissected, or if sections are made of such a
gland without being treated with acid, lamell like those in the
posterior glands and coated with cellular matter could be plainly seen,
together with a multitude of free calciferous cells readily soluble in
acetic acid. When a gland is completely filled with a single large
concretion, there are no free cells, as these have been all consumed in
forming the concretion. But if such a concretion, or one of only
moderately large size, is dissolved in acid, much membranous matter is
left, which appears to consist of the remains of the formerly active
lamell . After the formation and expulsion of a large concretion, new
lamell must be developed in some manner. In one section made by my son,
the process had apparently commenced, although the gland contained two
23