Earthworm is the common name for the largest members of Oligochaeta (which is
either a class or subclass depending on the author) in the phylum Annelida.

Earthworms are also called megadriles (or big worms), as opposed to the microdriles
(or small worms) in the families Tubificidae, Lumbriculidae, and Enchytraeidae, among
others.

Scientific classification
Kingdom: Animalia

Phylum: Annelida

Class: Clitellata

Order: Haplotaxida

Suborder: Lumbricina

Behavior

Earthworms are seen on the surface after heavy rain storms flood the soil because,
despite needing a moist environment to allow the diffusion of gases across their skin
membrane, where the soil becomes saturated they begin to drown. To protect
themselves they escape to the surface, but if the ground is un-naturally hard they may
later become stranded and die from exposure. This is why they are seen in places like
driveways after a storm. However, this theory is not applicable to certain earthworm
species that can survive immersion for several days in oxygenated water.


An alternative theory concerning this behavior is that as some species (notably
Lumbricus terrestris) come to the surface to mate they may become stranded.
However, as this behaviour is limited to only a few species and L. terrestris is rarely, if
ever, one of those found stranded on impermeable surfaces, this theory does not seem
a very likely explanation.

Another theory is that the worms may be using the moist conditions on the surface to
travel more quickly than they can underground, thus colonizing new areas more quickly.
Since the relative humidity is higher during and after rain, they do not become
dehydrated. This is a dangerous activity in the daytime, since earthworms die quickly
when exposed to direct sunlight with its strong UV content, and are more vulnerable to
predators such as birds.

A further theory is that, as there are many other organisms in the ground as well, and
their respiration increases carbon dioxide, this gas may dissolve into the rainwater to
form carbonic acid. As the soil becomes too acidic for the worms, they seek a more
neutral environment on the surface.

Locomotion and importance to soil

Close up of an earthworm in garden soil Earthworms travel underground by the means
of waves of muscular contractions which alternately shorten and lengthen the body. The
shortened part is anchored to the surrounding soil by tiny claw-like bristles (setae) set
along its segmented length. (In all the body segments except the first, last and clitellum,
there is a ring of S- shaped setae, embedded in the epidermal pit of each
segment,perichaetine) The whole burrowing process is aided by the secretion of
lubricating mucus. Worms can make gurgling noises underground when disturbed as
a result of the worm moving through its lubricated tunnels. They also work as biological
"pistons' forcing air through the tunnels as they move. Thus earthworm activity aerates
and mixes the soil, and is constructive to mineralization and nutrient uptake by
vegetation. Certain species of earthworm come to the surface and graze on the higher
concentrations of organic matter present there, mixing it with the mineral soil. Because
a high level of organic matter mixing is associated with soil fertility, an abundance of
earthworms is beneficial to the organic gardener. In fact as long ago as 1881 Charles
Darwin wrote: It may be doubted whether there are many other animals which have
played so important a part in the history of the world, as have these lowly organized
creatures.

Source: Wikipedia
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