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8.4. Annelids

Phylum Annelida

The annelids are a group animals invertebrates soft body, cylindrical and elongated, characterized by being divided into rings called metamers.

The body of the annelids is made up of numerous metamers or rings similar to each other. The internal anatomy of the annelids is a reflection of the external, with repetition of various organs in each metamere.

Although the annelids are sometimes referred to as worms, this is not correct since other very different animal phyla could be included within the worms, such as the flatworms or the nematodesThese phyla share characteristics, such as their bilateral symmetry, not having jointed appendages, and living in humid environments.

Los gusanos incluyen a los anélidos, platelmintos y nematodos

Most of the annelids are aquatic (such as the leech), although they can also be terrestrial (such as the earthworm), and free-living. There are also some that are parasites.

Their respiration is gill or cutaneous, through the skin, so they have to live attached to water or in moist soil.

Annelids are more complex than porifera and cnidarians, as they have tissues and apparatus such as the digestive (with a mouth and anus), the circulatory (closed), and the excretory.

Bilateral symmetry appears in these organisms , being able to divide the body longitudinally into two symmetrical parts.

The process of cephalization begins, in which the mouth, sense organs, and nerve ganglia are grouped together at the front of the body.

They have sexual reproduction (with eggs and sperm), and although there are hermaphrodite species, they do not usually self-fertilize. With cross fertilization, gametes are exchanged and the variability of the species is favored. They are oviparous beings.

Three groups of annelids are distinguished:

With the exception of hirudíneos (leeches), which are parasites and feed on the blood of other organisms, the rest of the annelids are characterized by having setae. The setae are hairy structures of chitin with tactile and locomotive function. The setae are very abundant in the polychaete (eg Nereis) and less abundant in oligochaetes (for example, earthworms).

Class Polychaetes

As its name indicates, the Polychaetes Class is a group of invertebrate animals of the Phylum Annelida characterized by having "many setae". The setae allow these organisms to crawl. Its body is made up of rings and its head is differentiated.

Your way of life can be:

  • Of free life. For example, the Nereis, formed by a body divided into rings with expansions with which they can crawl.


  • Sedentary. For example, tube worms that generate hard tubes in which they hide to protect themselves from predators. The gills go through these tubes to get oxygen from the water. Their appearance is very striking and that is why they are also called "duster worms".

Class Oligochaetes

The oligochaetes are a class of annelids with the body divided into rings but, unlike the polychaetes, they have few and small setae. Its body is elongated, cylindrical, and with well-differentiated ends. At the front is the head with the mouth, and at the back, the anus.

An example of an oligochaete is the earthworm, one of the few terrestrial annelids, although it needs a moist environment. The earthworm digs its galleries in the moist soil and engulfs the earth along with the organic matter on which they feed. Their body only keeps the food and they return the soil, so they aerate the soil and are beneficial.

Class Hirudineos

Hirudineans (Hirudinea)are a class of the phylum annelids, popularly known as leechesThere are marine, terrestrial, and arboreal hirudines , but the vast majority of species are freshwater. All leeches are hermaphrodites. 

Hirudine are annelids that lack setae and have suckers. They are ectoparasites (external parasites) that feed on the blood of vertebrates. Leeches are attached to the skin of an animal by two suckers, one anterior and one posterior. In the anterior sucker, it opens its mouth and sucks a little blood after the wound. The saliva of the leech contains a substance that anesthetizes the parasitized animal and makes it not notice the wound. In addition, saliva contains a protein called hirudin that prevents the blood from coagulating and can continue to absorb it.

Formerly, leeches were used in medicine during surgery to prevent the accumulation of blood around a muscle.

Sanguijuela medicinal (Arhynchobdellidae)

By GlebK [CC BY-SA 3.0 ], from Wikimedia Commons