按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
we know from Fredericq that a secretion of this kind is found in
the intestines of worms。 As the leaves which are dragged into the
burrows are often dry and shrivelled； it is indispensable for their
disintegration by the unarmed mouths of worms that they should
first be moistened and softened； and fresh leaves； however soft and
tender they may be； are similarly treated； probably from habit。
The result is that they are partially digested before they are
taken into the alimentary canal。 I am not aware of any other case
of extra…stomachal digestion having been recorded。 The boa…
constrictor is said to bathe its prey with saliva； but this is
doubtful； and it is done solely for the sake of lubricating its
prey。 Perhaps the nearest analogy may be found in such plants as
Drosera and Dionaea； for here animal matter is digested and
converted into peptone not within a stomach； but on the surfaces of
the leaves。
Calciferous Glands。These glands （see Fig。 1）； judging from their
size and from their rich supply of blood…vessels； must be of much
importance to the animal。 But almost as many theories have been
advanced on their use as there have been observers。 They consist
of three pairs； which in the common earth…worm debouch into the
alimentary canal in advance of the gizzard； but posteriorly to it
in Urochaeta and some other genera。 ｛23｝ The two posterior pairs
are formed by lamellae； which； according to Claparede； are
diverticula from the oesophagus。 ｛24｝ These lamellae are coated
with a pulpy cellular layer； with the outer cells lying free in
infinite numbers。 If one of these glands is punctured and
squeezed； a quantity of white pulpy matter exudes； consisting of
these free cells。 They are minute； and vary in diameter from 2 to
6 microns。 They contain in their centres a little excessively fine
granular matter； but they look so like oil globules that Claparede
and others at first treated them with ether。 This produces no
effect； but they are quickly dissolved with effervescence in acetic
acid； and when oxalate of ammonia is added to the solution a white
precipitate is thrown down。 We may therefore conclude 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 lamellae； 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 1。5
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； lamellae 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 lamellae。 After the
formation and expulsion of a large concretion； new lamellae must be
developed in some manner。 In one section made by my son； the
process had apparently commenced； although the gland contained two
rather large concretions； for near the walls several cylindrical
and oval pipes were intersected； which were lined with cellular
matter and were quite filled with free calciferous cells。 A great
enlargement in one direction of several oval pipes would give rise
to the lamellae。
Besides the free calciferous cells in which no nucleus was visible；
other and rather larger free cells were seen on three occasions；
and these contained a distinct nucleus and nucleolus。 They were
only so far acted on by acetic acid that the nucleus was thus
rendered more distinct。 A very small concretion was removed from
between two of the lamellae within an anterior gland。 It was
imbedded in pulpy cellular matter； with many free calciferous
cells； together with a multitude of the larger； free； nucleated
cells； and these latter cells were not acted on by acetic acid；
while the former were dissolved。 From this and other such cases I
am led to suspect that the calciferous cells are developed from the
larger nucleated ones； but how this was effected was not
ascertained。
When an anterior gland contains several minute concretions； some of
these are generally angular or crystalline in outline； while the
greater number are rounded with an irregular mulberry…like surface。
Calciferous cells adhered to many parts of these mulberry…like
masses； and their gradual disappearance could be traced while they
still remained attached。 It was thus evident that the concretions
are formed from the lime contained within the free calciferous
cells。 As the smaller concretions increase in size； they come into
contact and unite； thus enclosing the now functionless lamellae；
and by such steps the formation of the largest concretions could be
followed。 Why the process regularly takes place in the two
anterior glands； and only rarely in the four posterior glands； is
quite unknown。 Morren says that these glands disappear during the
winter； and I have seen some instances of this fact； and others in
which either the anterior or posterior glands were at this season
so shrunk and empty； that they could be distinguished only with
much difficulty。
With respect to the function of the calciferous glands； it is
probable that they primarily serve as organs of excretion； and
secondarily as an aid to digestion。 Worms consume many fallen
leaves； and it is known that lime goes on accumulating in leaves
until they drop off the parent…plant； instead of being re…absorbed
into the stem or roots； like various other organic and inorganic
substances。 ｛25｝ The ashes of a leaf of an acacia have been known
to contain as much as 72 per cent。 of lime。 Worms 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。