the annelids are animals with body nearly
cylindrical rings or transverse folds. These rings or folds are external, but
correspond to internal segments of the body. Annelids have a cavity called coelom,
which is divided by transverse partitions. Inside the coelom, there is a fluid
in which the organs are suspended.
The mentioned transverse partitions separate the different segments of the
body, which have some portion of the nervous system and the circulatory
system. Therefore, they can function almost independently. Each of the segments
is known as a metamer and is marked, on the outside, by one or
Specialists estimate that there are about 16,500 species of
annelids, which can reach a length of between less than a millimeter and more
than three meters. Most live in the marine environment, although there are some
species that reside on land (such as the earthworm)
and others that live in fresh water (the leech).
Annelids belong to the worm type, although not all worms are annelids. In
this case, these are vermiform protostomous invertebrate animals.
When classifying annelids it is important to underline that basically four
main groups are established:
Citelated. These are the living beings that are characterized because they
have what is known as clitellus which is nothing more than a part of their body
that is basically functioning within what are the tasks of reproduction. Within
this large group we would also have to highlight that it is subdivided into two
groups: the hirudíneos, such as leeches, and the oligoquetos, which includes
species such as the known earthworms.
Equiuroids. Basically within this large group we would highlight that it is
made up of those marine-type worms that have a tube that cannot move and that
are not segmented.
Polychaetes. This third group of annelids we can emphasize that it is the
largest of the four classifications and it is essentially made up of those that
are marine. They also stand out for the fact that they use parapods to dig or
swim, among other actions.
Mizostomids. To speak of this last set is to emphasize that it is made up of
everything that is the parasites of echinoderms.
As we mentioned earlier, the best known annelids are worms, earthworms, and
leeches. The latter are characterized, above all, by the fact that they are
aquatic, fundamentally they live in fresh waters, and that they have an
approximate length of about twelve centimeters. Another of their hallmarks is
that they feed on the blood they suck from the living things they cling to.
The exterior of the annelids is covered by an epidermis that
secretes a protective cuticle. Beneath the epidermis is a layer of circular
musculature that, in turn, covers a layer of longitudinal muscle fibers.
Finally, it should be noted that annelids can reproduce both sexually (some
species are hermaphrodites) and asexually (by the fission