Segmented worms with rounded (earthworm) or flattened (leech) bodies,
and possessing both mouth and anus. They fall into two sub-classes: I.
Leeches (_Discophora_), II. Bristle-worms (_Chætopoda_). The first
possess suckers as organs assisting locomotion, the latter foot-stumps,
which bear longer or shorter bristles; but these foot-stumps may be
absent, and the bundles of bristles are then simply imbedded in pits.
The latter is the case in the only members of the class which interest
us here, _i.e._ the _earthworms_.
The =Earthworms= (_Lumbricus_).
There are several species in the genus, but all agree in their habits.
The body of the earthworm is slender and cylindrical, tapering in front
and somewhat flattened behind. On the ventral side of the second segment
is found the opening of the mouth. There are no eyes, though these
animals are sensitive to the action of light. The earthworm is
hermaphrodite (p. 16); when two individuals pair they mutually fertilize
each other. On a warm summer evening two adjacent worms creep half out
or nearly out of their burrows, and apply the front parts of their
bodies together, especially a reddish, swollen part found at about the
middle of the anterior half of the body. In this region the glands in
the skin are very strongly developed, and secrete a substance which
surrounds the eggs as a capsule as they are being laid. Earthworms are
chiefly found in damp humus, or, at any rate, not in very poor sandy
soil or clay. From time to time they carry their burrows up to the
surface in order to get rid of the undigested remains (“worm castings”)
of the humus and vegetable matters which have been taken into the body.
The burrows run down obliquely into the soil, or more rarely vertically,
to the depth of eight feet or more; they end in an enlargement, where
the worm remains coiled up during the winter, after having closed the
mouth of the burrow with a plug of leaves, twigs, paper, straw, etc.
Although the earthworm chiefly subsists on the organic matters found in
earth rich in humus, it also devours the leaves of cabbage, onion, and
other plants, and especially seedlings (particularly those of beet). It
draws these parts of plants about four-fifths of an inch into its
burrow, and moistens them there with an acid fluid it secretes, and
which acts upon them before they are taken into the body. Earthworms may
effect considerable damage by destroying seedlings, particularly in damp
fields, though this damage is always local. A not inconsiderable amount
of benefit is to be set against this. By means of the burrows which they
dig earthworms cause air to penetrate into the soil much better than it
could otherwise do, which is known to be of the greatest importance for
plant life. Earthworms are extremely important owing to the large
numbers in which they are present in the soil, and for another reason
besides the one just given. Darwin shows that in many parts of England a
weight of ten tons of earth per acre passes through the bodies of
earthworms, and is brought to the surface by them, so that in a few
years the entire humus-containing surface layer of earth has passed
through their bodies. They therefore prepare the soil in an excellent
manner for the growth of plants, by continually exposing it to the air.
They cause stones to sink in the soil by throwing out at the top earth
which naturally consists only of particles small enough to pass through
their intestines. They play the part of gardeners by thoroughly mixing
together the particles of soil, and bury under their castings, in a
shorter time than one would imagine, objects found on the surface of the
soil (bones, oyster-shells, dead animals, leaves). Plantfood is quickly
formed again from these buried matters. Earthworms, therefore, play a
very important part in the economy of nature; indeed, many regions, now
occupied by luxuriant pastures or fertile cornfields, would be a waste,
had it not been for them. Where, however, in damp spots they injure
seedlings, they may be collected, either during the day after a warm
rain, or in the evening, at which times they lie half out of their
burrows. If a decoction of walnut leaves is poured into these, they will
crawl out. _Enemies_: Mole, shrews, hedgehog, toads and frogs, ground
beetles, rove beetles, mole-crickets, centipedes, etc.