ature of the sites inhabited-Can live long under water-Nocturnal-
Wander about at night-Often lie close to the mouths of their burrows,
and are thus destroyed in large numbers by birds-Structure-Do not pos-
sess eyes, but can distinguish between light and darkness-Retreat rapidly
when brightly illuminated, not by a reflex action-Power of attention-
Sensitive to heat and cold-Completely deaf-Sensitive to vibrations and to
touch-Feeble power of smell-Taste-Mental qualities-Nature of food-
Omnivorous-Digestion-Leaves before being swallowed, moistened with
a fluid of the nature of the pancreatic secretion-Extra-stomachal digestion-
Calciferous glands, structure of-Calcareous concretions formed in the an-
terior pair of glands-The calcareous matter primarily an excretion, but sec-
ondarily serves to neutralise the acids generated during the digestive process.
EARTH-WORMS are distributed throughout the world under the form
of a few genera, which externally are closely similar to one another. The
British species of Lumbricus have never been carefully monographed; but
we may judge of their probable number from those inhabiting neighbouring
3 CHARLES DARWIN
countries. In Scandinavia there are eight species, according to Eisen; {8a} but
two of these rarely burrow in the ground, and one inhabits very wet places or
even lives under the water. We are here concerned only with the kinds which
bring up earth to the surface in the form of castings. Hoffmeister says that
the species in Germany are not well known, but gives the same number as
Eisen, together with some strongly marked varieties. {8b}
Earth-worms abound in England in many different stations. Their castings
may be seen in extraordinary numbers on commons and chalk-downs, so as
almost to cover the whole surface, where the soil is poor and the grass short
and thin. But they are almost or quite as numerous in some of the London
parks, where the grass grows well and the soil appears rich. Even on the same
field worms are much more frequent in some places than in others, without
any visible difference in the nature of the soil. They abound in paved court-
yards close to houses; and an instance will be given in which they had bur-
rowed through the floor of a very damp cellar. I have seen worms in black
peat in a boggy field; but they are extremely rare, or quite absent in the drier,
brown, fibrous peat, which is so much valued by gardeners. On dry, sandy
or gravelly tracks, where heath with some gorse, ferns, coarse grass, moss and
lichens alone grow, hardly any worms can be found. But in many parts of
England, wherever a path crosses a heath, its surface becomes covered with a
fine short sward. Whether this change of vegetation is due to the taller plants
being killed by the occasional trampling of man and animals, or to the soil
being occasionally manured by the droppings from animals, I do not know.
{9b} On such grassy paths worm-castings may often be seen. On a heath
in Surrey, which was carefully examined, there were only a few castings on
these paths, where they were much inclined; but on the more level parts,
where a bed of fine earth had been washed down from the steeper parts
