Biology is something extremely beautiful and perfect , it seems something conceived by a magnificence. There are things like annelids , which turn out to be quite unpleasant to look at, these in the same way are something incredible and necessary for nature , since they fulfill an important function in the environment.


What are annelids?

They are crudely known as worms which have tubular or worms divided but actually preceding a Annelida edge. Annelids are invertebrate animals , because there are worms that derive from different animal phyla. Its name comes from the characteristic of its most predominant body , the rings .

  • Characteristics of annelids
  • Classification
  • Annelids feeding
  • Nutrition
  • Annelid circulatory system
  • Breathing
  • Reproduction
  • Importance of annelids

Characteristics of annelids

Among the main characteristics of annelids we find:

  • They are approximately 1 micrometer or millimeter to 1 meter in size.
  • They are soft bodied .
  • They lack a skeleton .
  • They present bilateral symmetry .
  • They move by coordinating the contractions of your muscles.
  • Most have filaments called silks or quetas.
  • The habitat where they can be found is on land (worms), others in fresh water (leeches) and others in the sea .
  • In turn, some live freely and there are others that are fixed in a single place such as the bottom of the sea or rocks, they even stick to other animals.
  • The leech is the parasite that gives them to mammals .


There are several classifications but there are only two divisions that are the largest extensive worldwide and they are:

Polychaeta or Polychaetes

They have more than 10,000 species that makes the classification with more species, most are marine and can live freely swimming and what stands out the most in them is that it shows two paropodia , that is, different supplements with various ketes . In general they are carnivores while others feed on sediments. An example for this classification would be for Spirobranchus giganteus, which is the Christmas tree worm and the Miñoca.

Clitellata or Clitelados

They are easy to recognize because they present as a kind of necklace that it has a purpose for reproduction that is part of its life cycle. These are divided into two more subclasses:

  • Oligochaetes: they are terrestrial animals, most of us can use the common Earthworm as an example, it does not have parapodia but it does have tiny quetas, even a few.
  • Hirudíneos: we put Leeches as the first example, they live most in environments with salt water, they are like predators of invertebrate animals smaller than them but there are also some that feed on the blood of vertebrate animals.

Annelids feeding

The diet  varies according to the species and what each one is used to, for example leeches are used to feeding on the blood of mammals while there is an aquatic group that only feeds on seedlings, others that are terrestrial and feed on the nutrients of the earth.


Annelid digestive system:

It is tube-shaped and has 2 openings in which two lips appear,  pharynx that is to swallow food , the muscular one, the crop to store it, the gizzard that is for grinding sand grains or any food consumed . The intestine that runs throughout the body until it reaches the anus .

  • Digestion:  This process occurs when food is eaten or swallowed , and passes through the pharynx, which is responsible for moistening the secretions of the glands in the walls, immediately afterwards and passes into the esophagus where calciferous glands are found and these neutralize all acidity From there, the crop is the one that stores and the gizzard crushes it so that it concludes in the intestine where the digestion cycle is totalized.
  • Absorption: Substances that have been absorbed pass into the circulatory stream and there the waste is expelled through the anus.

Annelid circulatory system

There are open and closed circulatory systems:

  • The blood is completely separated from the fluid in the coelom , being transported by blood vessels through the segments and septa
  • Main Blood Vessels : One runs through the ventral area , another through the dorsal area . And these vessels pass the blood along the sides of the worm and create vesicles or hearts that give impulse to the blood.


In general, annelids  breathe through the skin , called cutaneous respiration. The worms for example always noticed with wet skin  due to the mucous substance coating the body because they otherwise could not breathe.

Those that exist in the water breathe through the gills where they capture oxygen from the water they conserve, such as ocean worms that are used as bait for fishing.


Annelids are reproduced so sexual or asexual everything is dependent on the species . Some species have only one of the types of reproduction while others combine both.

In the case of the Asexual there is a group that can do them by budding, which is when they generate a new individual united to the other, in the other case there are those that do it through fragmentation , that is, they fragment themselves into two or more new individuals.

Many of the annelids are androgynous and more than all they are seen in the marine ones where their process of reproduction is when one begins to release semen while another releases ovules or vice versa and finally procreation happens externally and the eggs that were fertilized usually lodge. most of the time as Plakton until gradually they progress to the base of the sea there they undergo a kind of metamorphosis and become premature adults.

Importance of annelids

The annelids are of utmost importance for the ecological area, since the continuous excavation in the soils helps and allows the aeration of the earth , they are extremely fundamental for the maintenance of the soil functions and increase the carbon stock.

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