have been seen to be timid creatures. Their eagerness for certain
kinds of food manifestly shows that they must enjoy the pleasure of
eating. So strong is their sexual passion that they overcome for a time
their dread of light. They seem to have a trace of social feeling, for
they are not disturbed by crawling over each other’s bodies, and they
sometimes lie in contact. Although remarkably deficient in the several
sense-organs, yet this does not necessarily preclude intelligence, for
it has been shown that when their attention is engaged they neglect
impressions to which they would otherwise have attended, and attention,
as is well known, indicates the presence of a mind of some kind. A few
actions are performed instinctively, that is, all the individuals,
including the young, perform each action in nearly the same manner.
The various species of Perichæta eject their castings so as to
construct towers, and the burrows of the Common Earth-worm--_Lumbricus
terrestris_--are smoothly lined with fine earth and often with little
stones, and the mouth with leaves. One of their strongest instincts is
the plugging up of the mouths of their burrows with various objects,
the very young worms acting in a similar manner. But some degree of
intelligence is manifested, as will subsequently appear.
Almost everything is eaten by worms. They swallow enormous quantities
of earth, from which they extract any digestible matter it may
contain. Large numbers of half-decayed leaves of all kinds, excepting
a few that are too tough and unpleasant to the taste, and likewise
petioles, peduncles, and decayed flowers. Fresh leaves are consumed
as well. Particles of sugar, licorice and starch, and bits of raw and
roasted meat, and preferably raw fat, are eaten when they come into
their possession, but the last article with a better relish than any
other substance given to them. They are cannibals to a certain extent,
and have been known to eat the dead bodies of their own companions.
The digestive fluid of worms, according to León Frédéricq, is analogous
in nature to the pancreatic secretion of the higher animals, and this
conclusion agrees perfectly with the kinds of food which they consume.
Pancreatic juice emulsifies fat, dissolves fibrin, and worms greedily
devour fat and eat raw meat. It converts starch into grape-sugar with
wonderful rapidity, and the digestive fluid of worms acts upon the
starch of leaves. But worms live chiefly on half-decayed leaves, and
these would be useless to them unless they could digest the cellulose
forming the cell-walls, for all other nutritious substances, as is well
known, are almost completely withdrawn from leaves shortly before they
fall off. It has been ascertained that cellulose, though very little
or not at all attacked by the gastric juice of the higher animals, is
acted on by that from the pancreas, and so worms eat the leaves as much
for the cellulose as for the starch they contain. The half-decayed or
fresh leaves which are intended for food are dragged into the mouths
of their burrows to a depth of from one to three inches, and are then
moistened with a secreted fluid, which has been assumed to hasten their
decay, but which, from its alkaline nature, and from its acting both
on the starch-granules and on the protoplasmic contents of the cells,
is not of the nature of saliva, but a pancreatic secretion, and of
the same kind as 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 the unarmed mouths of worms that they should
first be moistened and softened, their disintegration being thereby the
more readily effected. Fresh leaves, however soft and tender they may
be, are similarly treated, probably from habit. Thus the leaves are
partially digested before they are taken into the alimentary canal, an
instance of extra-stomachal digestion, whose nearest analogy is to be
found in such plants as Dionæa and Drosera, for in them animal matter
is digested and converted into peptone, not within a stomach, but on
the surfaces of the leaves.
But no portion of the economy of worms has been more the subject of
speculation than the calciferous glands. About as many theories have
been advanced on their utility as there have been observers. Judging
from their size and from their rich supply of blood-vessels, they must
be of vast importance to these animals. They consist of three pairs,
which in the Common Earth-worm debouch into the alimentary canal in
front of the gizzard, but posteriorly to it, in some genera. The two
posterior pairs are formed by lamellæ, diverticula from the œsophagus,
which 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, which are minute bodies, varying in diameter from
two to six millimetres. They contain in their centres a small quantity
of excessively fine granular matter, that looks so like oil globules
that many scientists are deceived by its appearance. When treated with
acetic acid they quickly dissolve with effervescence. An addition of
oxalate of ammonia to the solution throws down a white precipitate,
showing that the cells contain carbonate of lime. The two anterior
glands differ a little in shape from the four posterior ones by being
more oval, and 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 one and one-half millimetres in diameter.
With respect to the function of the calciferous glands, it is likely
that they primarily serve as organs of excretion, and secondarily as
an aid to digestion. Worms consume many fallen leaves. 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, and worms would
therefore be liable to become charged with this earth, unless there
was some special apparatus for its excretion, and for this purpose the
calciferous glands are ably adapted. On the other hand, 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. These half-decayed leaves, which are
swallowed by worms in large quantities, would, therefore, after having
been moistened and triturated in the alimentary canal, be apt to
produce such acids, and in the case of several worms, whose alimentary
canals were examined, their contents were plainly shown by litmus
paper to be decidedly acid. This acidity cannot be attributed to the
nature of the digestive fluid, for pancreatic juice is alkaline,
and so also is the secretion which is poured out of the mouths of
worms for the preparation of the leaves for consumption. With worms
not only the contents of the intestines, but their ejected matter
or the castings are generally acid. The digestive fluid of worms
resembles in its action, as already stated, the pancreatic secretion
of the higher animals, and in these latter pancreatic digestion is
necessarily alkaline, and the action will not take place unless some
alkali be present; and the activity of an alkaline juice is arrested
by acidification, and hindered by neutralization. Therefore is seems
probable that innumerable calciferous cells, which are emptied from
the four posterior glands in the alimentary canal, serve to neutralize
more or less completely the acids generated there by the half-decayed
leaves. These cells, as has been seen, are instantly dissolved by a
small quantity of acetic acid, and as they do not always suffice to
render of no effect the contents of the upper part of the alimentary
canal, it is probable that the lime is aggregated into concretions,
in the anterior pair of glands, in order that some may be conveyed to
the posterior parts of the intestine, where these concretions would
be rolled about among the acid contents. The concretions found in the
intestines and in the castings often present a worn appearance, but
whether due to attrition or chemical corrosion it is impossible to
say. That they are formed for the sake of acting as mill stones, as
Claparède believed, and of thus assisting in the trituration of food,
is not at all likely, as this object is already attained by the stones
that are present in the gizzards and intestines.
In dragging leaves into their burrows worms generally seize the thin
edge of a leaf with their mouths, between the projecting upper and
lower lip, the thick and strong pharynx at the same time being pushed
forwards within their bodies, so as to afford a _point de resistance_
for the upper lip; but in the case of broad and flat objects the
pointed anterior extremity of the body, after being brought into
contact with an object of this kind, is drawn within the adjoining
rings, so that it becomes truncated and as thick as the rest of the
body. This part is then seen to swell a little, seemingly from the
pharynx being pushed a little forwards. By a slight withdrawal of
the pharynx, or by its expansion, a vacuum is produced beneath the
truncated, slimy end of the body whilst in contact with the object,
and by this means the two adhere firmly together. Worms can attach
themselves to an object in the same manner under the water.
[Illustration: COMMON EARTH-WORMS.
Out on a Foraging Excursion.]
As worms have no teeth, and their mouths consist of very soft tissue,
it may be presumed that they consume by means of suction of the edges
and parenchyma of fresh leaves after they have been softened by the
digestive fluid. They cannot attack such strong leaves as those of
sea-kale or large and thick leaves of ivy. They not only seize leaves
and other objects for purposes of food, but for plugging up the mouths
of their burrows. Flower-peduncles, decayed twigs of trees, bits of
paper, feathers, tufts of wool and horse-hair are some of the many
things other than leaves that are dragged into their burrows for this
purpose. Many hundred leaves of the pine-tree have been found drawn by
their bases into burrows. Where fallen leaves are abundant, especially
ordinary dicotyledonous leaves, many more than can be used are
collected over the mouth of a burrow, so that a small pile of unused
leaves is left like a roof over those which have been partly dragged
in. A leaf in being dragged a little way into a cylindrical burrow
necessarily becomes much folded or crumpled, and when another is drawn
in, this is done exteriorly to the first, and so on with succeeding
leaves, till finally they all become closely folded and pressed
together. Sometimes the mouth of a burrow is enlarged, or a fresh one
is made close by, so that a larger number of leaves may be drawn in.
Generally the interstices between the drawn-in leaves are filled with
moist, viscid earth ejected from their bodies, thus rendering them
doubly secure. Hundreds of such plugged burrows may be seen during the
autumnal and early winter months.
When leaves, petioles, sticks, etc., cannot be obtained for the mouths
of their burrows, heaps of stones, smooth, rounded pebbles, are
utilized for protection. When the stones are removed and the surface
of the ground is cleared for some inches round the burrow, the worms
may be seen with their tails fixed in their burrows dragging the
stones inward by the aid of their mouths, stones weighing as much as
two ounces often being found in the little heaps, which goes to show
how strong these apparently weak creatures are. Work of this kind
is usually performed during the night, although objects have been
occasionally known to be drawn into the burrows during the day. What
advantage worms derive from plugging up the mouths of their burrows,
or from piling stones over them, cannot be satisfactorily answered.
They do not act in this manner when they eject much earth from their
burrows, for then their castings serve to cover the mouth. Perhaps the
plugs serve to protect them from the attacks of scolopenders, their
most inveterate enemies, or to enable them to remain with safety with
their heads close to the mouths of their burrows, which they like so
well to do, but which, unless protected, costs many a fellow its life.
Besides, may not the plugs check the free ingress of the lowest stratum
of air, when chilled by radiation at night, from the surrounding ground
and herbage? The last view of the matter seems especially well taken,
because worms kept in pots where there is fire, having no cold air
with which to contend, plug up their burrows in a slovenly manner, and
because they often coat the upper part of their burrows with leaves,
apparently to prevent their bodies from coming into contact with the
cold, damp earth. But the plugging-up process may undoubtedly serve
for all these purposes. Whatever the motive may be, it seems that
worms much dislike leaving the mouths of their burrows open, yet,
nevertheless, they will reopen them at night, whether or not they are
able afterwards to close them.
Considerable intelligence is shown by worms in their manner of plugging
up their burrows. If man had to plug up a cylindrical hole with such
objects as leaves, petioles or twigs, he would push them in by their
pointed ends, but if these were thin relatively to the size of the
hole, he would probably insert some by their broader ends. Intelligence
would certainly be his guide in such a case. But how worms would drag
leaves into their burrows, whether by their tips, bases, or middle
parts, has been a matter of interest to many. Darwin, who experimented
upon the subject, found it especially desirable to experiment with
plants not native to his country, for he conceived that although the
habit of dragging leaves into their burrows is undoubtedly instinctive
with worms, yet instinct could not teach them how to act in the case of
leaves about which their progenitors knew nothing. Did they act solely
through instinct, or an unvarying inherited impulse, they would draw
all kinds of leaves into their burrows in the same manner. Having no
such definite instinct, chance might be expected to determine whether
the tip, base, or middle might be seized. If the worm in each case
first tries many different methods, and follows that alone which proves
possible or the most easy, then both instinct and chance are ruled out
of the solution of the question. But to act in this manner, and to try
different methods, makes what in man would be called intelligent action.
Three species of pine-leaves are mentioned by Darwin as being regularly
drawn into the mouths of worm-burrows on the gravel-walk in his garden.
These leaves consist of two needles, which are united to a common base,
and it is by this point that they are almost invariably drawn into
the burrows. As the sharply-pointed needles diverge somewhat, and
as several are drawn into the same burrow, each tuft forms a perfect
_chevaux-de-frise_. Many tufts were pulled up in the evening, but
by the ensuing morning fresh leaves had taken their places, and the
burrows again well protected. Impossible it would be to drag these
leaves to any depth into the burrows, except by their bases, as a
worm cannot seize hold of the two leaves at the same time, and if one
alone were seized by the apex, the other would be pressed against the
ground and resist the entry of the one that was seized. That the worms
should do their work well, it was very essential that they drag the
pine-leaves into their burrows by their bases, that is, where the two
needles are conjoined. But how they are guided in this work was at
first perplexing. The difficulty, however, was soon settled. With the
assistance of his son Francis, the elder Darwin set to work to observe
worms in confinement during several nights by the aid of a dim light,
while they dragged the leaves of the aforementioned kinds into their
burrows. They were seen to move the anterior extremities of their
bodies about the leaves, and on several occasions when they touched
the sharp end of the needle they suddenly withdrew as though they had
been pricked, but it is doubtful that they were hurt, for they are
indifferent to sharp objects, being known to swallow rose-thorns and
small splinters of glass. It may be doubted whether the sharp end of
the needle serves to tell them that is the wrong end to seize, for the
points of many were cut off for the length of an inch, and these leaves
were always drawn in by their bases and not by the cut-off ends. The
worms, it seemed, almost instantly perceived as soon as they had seized
a leaf in the proper manner. Many leaves were cemented together at the
top, or tied together by fine thread, and these in the majority of
instances were dragged in by their bases, which leads to the conclusion
that there must be something attractive to worms in the base of
pine-leaves, notwithstanding that few ordinary leaves are drawn in by
their base or footstalk. Leaves of other plants, and also the petioles
of some compound plants, as well as triangular bits of paper, dry and
damp, were experimented with, and the manner of seizing the objects and
bearing them into their burrows were as amusing as they were novel and
interesting. The leaves and stems used were such as the worms had not
been accustomed to in their respective haunts.
When the several cases experimented on are considered, one can hardly
escape from the conclusion that some degree of intelligence is shown
by worms in plugging up their burrows. Each particular object is
seized in too uniform a manner, and from causes which we can generally
understand, for the result to be attributed to mere chance. That every
object has not been drawn in by its pointed end may be accounted for
by labor having been saved by some being carried in by their broader
ends. There is no doubt that worms are governed by instinct in plugging
up their burrows, and it might be expected that they would have been
taught in every particular instance how to act independently of
intelligence. It is very difficult to judge when intelligence comes
into play. The actions of animals, appearing due to intelligence,
may be performed through inherited habit without any intelligence,
although aboriginally acquired, or the habit may be acquired through
the preservation and inheritance of some other action, and in the
latter case the new habit will have been acquired independently of
intelligence throughout the entire course of its development. There is
no _à priori_ improbability in worms having acquired special instincts
through either of these two latter means. Nevertheless it is incredible
that instincts should have been developed in reference to objects, such
as the leaves and petioles of foreign plants, wholly unknown to the
progenitors of the worms which have acted in the manner just described.
Nor are their actions so unvarying or inevitable as are most true
instincts.
As worms are not controlled by special instincts in each particular
case, though possessing a general instinct to plug up their burrows,
and as chance is excluded, the next most probable conclusion is that
they try in many ways to draw in objects and finally succeed in some
one way. It is surprising, however, that an animal so low in the scale
as a worm should have the capacity to act in this way, as many higher
animals have no such capacity, the instincts of the latter often being
followed in a senseless or purposeless manner.
We can safely infer intelligence, as Mr. Romanes, who has specially
studied animals, says, only when we see an individual profiting by his
own experiences. That worms are able to judge either before or after
having drawn an object close to the mouths of their burrows how best
to drag it in, shows that they must have acquired some notion of its
general shape. This they probably acquire by touching it in many places
with the anterior extremity of their bodies, which serves them as a
tactile organ. Man, even when born blind and deaf, shows how perfect
the sense of touch may become, and if worms, which also come into being
in the same condition, have the power of acquiring some notion, however
rude, of the shape of an object and their burrows, they deserve, it
must seem to every sensible mind, to be called intelligent creatures,
for they act in such a case in nearly the same manner as a man would
under similar circumstances. That worms, which stand so low in the
scale of organization, should possess some degree of intelligence,
will doubtless strike everyone as very improbable. It may be doubted,
however, whether we know enough about the nervous system of the lower
animals to justify our natural distrust of such a conclusion. With
regard to the small size of the cerebral ganglia, we would do well
to remember what a mass of inherited knowledge, with some power of
adapting means to an end, is crowded into the minute brain of a worker
ant.
Two ways are adopted by worms in excavating their burrows. Either the
earth is pushed away on all sides or it is swallowed by the animal.
In the former case the worm inserts the stretched-out and attenuated
anterior extremity of its body into any little crevice or hole, and the
pharynx is pushed forward into this part, which consequently swells
and pushes away the earth on all sides, the anterior extremity thus
acting as a wedge. When placed in loose mould a worm will bury itself
in between two and three minutes, but in earth that is moderately
pressed down it often requires as many as fifteen minutes for its
disappearance. But whenever a worm burrows to a depth of several feet
in undisturbed compact ground, it must form its passage by swallowing
the earth, for it is impossible that the ground could yield on all
sides to the pressure of the pharynx when pushed forward within the
worm’s body. Great depths are reached only during continued dry weather
and severe cold, the burrows sometimes attaining to a depth of from
seven to eight feet. The burrows run down perpendicularly, or, more
commonly, obliquely, and are sometimes said to branch. Generally, or
invariably as I think, they are lined with fine, dark-colored earth
voided by the worm, so that at first they must be made a little wider
than their ultimate diameter. Little globular pellets of voided earth,
still soft and viscid, often dot the walls of fresh burrows, and these
are spread out on all sides by the worm as it travels up or down its
burrow, the lining thus formed becoming very compact and smooth when
nearly dry and closely fitting the worm’s body. Excellent points of
support are thus afforded for the minute reflexed bristles which
project in rows on all sides from the body, thus rendering the burrow
well adapted for the rapid movement of the animal. The lining appears
also to strengthen the walls, and perhaps saves the worm’s body from
being scratched, which would assuredly be the case when the burrows,
as is occasionally observed, pass through a layer of sifted coal
cinders. The burrows are thus seen to be not mere excavations, but
may be compared with tunnels lined with cement. Those which run far
down into the ground generally, or at least frequently, terminate in
little chambers, where one or several worms pass the winter rolled up
into a ball. Small pebbles and seeds as large as grains of mustard are
carried down from the surface by being swallowed or within the mouths
of worms, as well as bits of glass and tile, whose only use in their
winter-quarters seems to be the prevention of their closely coiled-up
bodies from coming into contiguity with the surrounding cold soil, for
such contact would perhaps interfere with their respiration, which is
effected by the skin alone.