According to DigoPaul, 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 more rings.
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 method).