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What are they?
Annelids
are a diverse group among the invertebrates. There are parasites and some are
even helpful. The leeches are the most famous ( or infamous ) of the parasites.
But the worms burrow through the soil. They rework the sediments that come in
their path ( that last line makes them seem more threatening than helpful ).
Characteristics
Their lengths range from less than an inch, to more than 5 metres. They generally have a head, trunk, and a tail. They may have gills - the marine ones atleast.
There is a fascinating feature of these animals. Though there are some parasites,
some exhibit occasional aggressive behavious, and some
are also able to produce light. Sometimes, this light
is mistaken for something supernatural ( and is, if you consider a worm
a supernatural being ). This is found as a sing of sexual maturity. They have
the ability to learn from their mistakes ( though, sometimes they don't get
a second chance when they go on ,what we would call a suicidal mission, of coming
next to their enemies ) .
Some of the worms reproduce sexually, some don't. In the marine ones, reproduction
is accomplished by shedding sperms and eggs into the water ( gametes ). When
they fertilize, they develop into diamond shaped larve, which later resemble
adults, when they lay settle on a suitable surface. Some of them can regenerate
body parts that are lost - mostly tails.
At sexual maturity, a thick structure at the centre known as clitellum
is formed. Annelids with a clitellum are under the class Clitellata
( the name speaks for itself ), and are sub divided into:
There is a group Polychaeta, that lack the clitellum, and have parapodia - a paddle like appendages, having many bristles. One group - parasitezes echinoderms, come under this group.
All leeches feed on blood. All are hermophroditic, and their reproduction is always sexual.
They are segmented into 34 portions, with suckers on either ends. Eggs are deposited in cocoons secreted by the clitellum, and the entire development of the young leech takes place inside this cocoon. The mode of locomotion consists in anchoring of a sucker, contraction of muscles, and then attachment of the other sucker.
The earthworm is very famous, it's helpful. They are extensive burrowers, and keep the soil irrigated. Their excreta is also good for the soil. A single square meter may have around 50 or more earthworms. Most of the earthworms and leeches are hermophrodites, having male and female organs.
The Archinannelida - consists of microscopic annelids. They seem to have lost many of their features that annelida's have - like segment, etc. They are not well known. They can also be known as the ancestors of all annelida's.
