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Earthworms have a closed circulatory system. They have two main
blood vessels that extend through the length of their body: a ventral
blood vessel which leads the blood to the posterior end, and a dorsal
blood vessel which leads to the anterior end. The dorsal vessel
is contractile and pumps blood forward, where it is pumped into
the ventral vessel by a series of "hearts" which vary
in number in the different taxa. A typical lumbricid will have 5
pairs of hearts. The blood is distributed from the ventral vessel
into capillaries on the body wall and other organs and into a vascular
sinus in the gut wall where gases and nutrients are exchanged.
One often sees earthworms come to the surface in large numbers
after a rainstorm. There are two theories for this behavior:
The first is that the waterlogged soil has insufficient oxygen
for the worms, therefore, earthworms come to the surface to get
the oxygen they need and breathe more easily. However, earthworms
can survive underwater for several weeks if there is oxygen in it,
so this theory is rejected by some.
Secondly, 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.
Earthworms are hermaphrodites (both female and male organs within
the same individual) but generally cannot fertilize their own eggs.
They have testes, seminal vesicles and male pores which produce,
store and release the sperm, and ovaries and ovipores. However,
they also have one or more pairs of spermathecae (depending on the
species) that are internal sacs which receive and store sperm from
the other worm in copulation. Copulation and reproduction are separate
processes in earthworms. The mating pair overlap front ends ventrally
and each exchanges sperm with the other. The cocoon, or egg case,
is secreted by the clitellum, the external glandular band which
is near the front of the worm, but behind the spermathecae. Some
indefinite time after copulation, long after the worms have separated,
the clitellum secretes the cocoon which forms a ring around the
worm. The worm then backs out of the ring, and as it does so, injects
its own eggs and the other worm's sperm into it. As the worm slips
out, the ends of the cocoon seal to form a vaguely lemon-shaped
incubator (cocoon) in which the embryonic worms develop. They emerge
as small, but fully formed earthworms, except for sexual structures,
which develop later. Some earthworm species are mostly parthenogenetic,
in which case the male structures and spermathecae may become abnormal,
or missing.
All text is available under the terms
of the GNU Free Documentation License
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