therefore would be liable to become charged with this earth, unless there
were some special means for its excretion; and the calciferous glands are
well adapted for this purpose. The worms which live in mould close over
the chalk, often have their intestines filled with this substance, and
their castings are almost white. Here it is evident that the supply of
calcareous matter must be super-abundant. Nevertheless with several
worms collected on such a site, the calciferous glands contained as many
free calciferous cells, and fully as many and large concretions, as did
the glands of worms which lived where there was little or no lime; and
this indicates that the lime is an excretion, and not a secretion poured
into the alimentary canal for some special purpose.
On the other hand, the following considerations render it highly probable
that the carbonate of lime, which is excreted by the glands, aids the
digestive process under ordinary circumstances. Leaves during their
decay generate an abundance of various kinds of acids, which have been
grouped together under the term of humus acids. We shall have to recur
to this subject in our fifth chapter, and I need here only say that these
acids act strongly on carbonate of lime. The half-decayed leaves which
are swallowed in such large quantities by worms would, therefore, after
they have been moistened and triturated in the alimentary canal, be apt
to produce such acids. And in the case of several worms, the contents of
the alimentary canal were found to be plainly acid, as shown by litmus
paper. This acidity cannot be attributed to the nature of the digestive
fluid, for pancreatic fluid is alkaline; and we have seen that the
secretion which is poured out of the mouths of worms for the sake of
preparing the leaves for consumption, is likewise alkaline. The acidity
can hardly be due to uric acid, as the contents of the upper part of the
intestine were often acid. In one case the contents of the gizzard were
slightly acid, those of the upper intestines being more plainly acid. In
another case the contents of the pharynx were not acid, those of the
gizzard doubtfully so, while those of the intestine were distinctly acid
at a distance of 5 cm. below the gizzard. Even with the higher
herbivorous and omnivorous animals, the contents of the large intestine
are acid. This, however, is not caused by any acid secretion from the
mucous membrane; the reaction of the intestinal walls in the larger as in
the small intestine is alkaline. It must therefore arise from acid
fermentations going on in the contents themselves . . . In Carnivora the
contents of the coecum are said to be alkaline, and naturally the amount
of fermentation will depend largely on the nature of the food. {49}
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