ANNELIDS: CHARACTERISTICS, TAXONOMY, REPRODUCTION, RESPIRATION - BIOLOGY - 2025

The annelids (Annelida) are a phylum of organisms that exhibit worm -like and live in humid environments, marine and freshwater land. This animal group includes the well-known leeches and earthworms, among other organisms called polychaetes, less known to man. This last grouping is the most diverse in the annelids.

The most important characteristic of the group is the presence of metamerism or segmentation of their bodies. Annelids have their bodies divided into a series of sections that are very similar to each other, where each of the parts has specialized structures for respiration, circulation, among others.

Earthworm, belonging to the Clitellates group.

They are distributed throughout the world and can feed on other invertebrates, decomposing organic matter or bodily fluids such as blood.

What is a worm?

In everyday life, we often use the term worm to refer to certain long, thin animals of varying sizes. However, the term groups together a series of invertebrates that are not related to each other. For this reason, the word worm has no taxonomic value.

The fact that several organisms present this vermiform structure may be due to a phenomenon known as evolutionary convergence, where organisms present similar characteristics but these do not imply a common origin.

Thus, it should be mentioned that not all worms belong to annelids. There are worm-shaped animals that are grouped into other phyla. For example, there are elongated, soft-bodied organisms that belong to nematodes, flatworms, or arthropods.

In other words, we should not instantly group a vermiform animal into the phylum Annelida.

General characteristics

Annelids are triploblastic animals with a body with bilateral symmetry. The coelom of these is schizocelic and is divided by septa, except in leeches.

The body is structured in very similar metamers, or rings. In fact, the group's name derives from the Latin roots annelus, which means little ring. There is usually a distinctive head.

The most popular forms are earthworms and leeches. There are also a number of marine worms, but the latter are not as well known.

The fluid that is contained within the coelom works as a hydrostatic skeleton.

This system works thanks to the constant volume of the fluid and the contraction of the metamers, which contain a series of muscles and allow the animals to move. The epithelium is characterized by secreting an external, moist and transparent cuticle.

They are found in very diverse habitats, from terrestrial and humid areas, to freshwater and marine bodies.

The ways of life are very heterogeneous within this animal group. Most forms are free-living, a few are symbiotic, and a much smaller number are ectoparasites of other organisms.

Morphology

The body of annelids is divided into segments called metamers, which are grouped in a linear series. On the outside, it is marked by a series of rings, a trait that gives the phylum its name.

In addition, the body is divided into an anterior region called the prostomy and peristomy, where all the structures related to the cephalic region are located.

Followed by this is the trunk, and the back is the pygidium. In this terminal region is the anus. Both the head and the pygidium are not considered segments.

The body of the animal grows in the following way: the new segments begin to differentiate just in front of the pygidium. Thus, the oldest segments are found in the anterior portion, while the oldest are in the posterior region of the worm.

Each segment has its own circulatory, respiratory and nervous systems. Likewise, each one has his coelom.

The coelom fluid acts as a hydrostatic skeleton. The volume of this does not change, so the muscular contraction of one area must be compensated by the dilation of another. In this way, the fluid provides a locomotion mechanism.

Taxonomy and classes

Annelids consist of more than 15,000 species of segmented worms. Most of the individuals are grouped in the class Polychaeta. In terms of appearance, they are quite heterogeneous, some could be considered grotesque under the popular conception, while others are graceful, delicate and colorful.

Present

In current taxonomy, Annelida includes pogonophores and dress worms. Previously, these organisms were grouped in the phylum Pogonophora, or they were grouped in different phyla: Pogonophora and Vestimentifera.

These organisms live in the sea, and usually inhabit deep regions. Today they are grouped within the class of polychaetes, in the clade Siboglinidae.

Related phyla

There are another series of organisms, very similar to the annelids, in terms of their vermiform morphology and their marine habits.

However, they do not have segmented bodies. These are the phyla Echiura and Sipuncula. Recent phylogenies that have been reconstructed using molecular data place the Echiura within the annelid phylum. According to this hypothesis, this group represents a polychaete where segmentation has been secondarily lost.

However, the phylogeny described with the inclusion of Echiura is not accepted by all expert annelid biologists.

Lessons

Traditionally, the annelid phylum has been divided into two classes: Polychaeta and Clitellata, in turn divided into the subclasses Oligochaeta and Hirudinea. We will describe each of the classes below:

Polychaeta class

With more than 10,000 species described to date, the class of polychaetes is considered the most diverse among the annelids. In addition to being very numerous, their colors are incredibly striking, with bright and iridescent colors.

They are divided into two groups, depending on their activity: sedentary and wandering or free-moving.

The length of these marine worms ranges from 5 to 10 cm. However, there are extremes: we have very small species, which are not larger than 1 mm, and individuals that manage to reach 3 meters.

Most of its members are capable of tolerating a wide range of salt concentrations. The fauna is usually more diversified in areas with warm temperatures.

They live in regions under rocks or in shells abandoned by their main hosts. They can also be found buried in the substrate. Others can build their own tubes and live in them.

They play a fundamental role in the food chains of aquatic ecosystems. They are prey to a number of marine animals, such as fish, crustaceans, hydroids, and others. In some regions of the world, they are considered a source of food for humans.

Class Clitellata

• Oligochaeta Subclass

Oligochaetes are a class of more than 3,000 species of organisms that appear in an enormous variety of forms and inhabit varied ecosystems. In this class are earthworms and other species that live in fresh water. Others are parasites or they live in the oceans.

The length of the animal is in a range from 12 to 30 centimeters. There are some exceptions that reach up to 4 meters in length, with more than 250 body segments.

They generally have nocturnal life habits, some may remain close to the surface of the ground and keep the anterior or terminal portion of the body out.

• Class Hirudinea

The hirudíneos are the leeches and animals related to them. "True" leeches have 34 body segments, lack ketae, and have a series of suction cups located in the anterior and posterior regions.

They generally inhabit bodies of fresh water, a few are marine and others have adapted to terrestrial ecosystems with high levels of humidity. They are particularly abundant in tropical countries, compared to their diversity in temperate zones.

Members of the order Acanthobdellida have 27 segments and have tags in the first five segments. The suction cup is located only in the posterior region.

Members of the order Branchiobdellida have 14 to 15 segments, do not have ketae and the vesicle is located in the anterior region. This order has commensal or parasitic life habits.

Relations between classes

The polychaetes are considered a paraphyletic group, since the ancestor of the oligochaetes and the hirudíneos arise within the polychaetes.

In contrast, the oligochaetes and hirudineans are grouped in the clade Clitellata. This group is characterized - as its name implies - by the presence of a specialized structure called the clitellus.

The clitellus is a ring or a glandular area that is located in a series of rings that vary in thickness. It is not present in the entire life of the animal, only in the reproductive season. It plays a role in reproduction, intervening in the union event of individuals during the sexual act.

Some authorities of the taxonomy and systematics of the annelids propose that Clitellata should be considered a class that contains the moated groups.

Reproduction

In polychaetes, the sexes are separated. These animals do not have permanent sexual organs. Thus, the reproductive system is quite simple. It is made up of the gonads that appear in the mating season. Fertilization is external and development occurs by means of a trochophore larva.

In contrast, oligochaetes are monoecious, that is, hermophrodites. In this way, both the male and female sexual organs are found in the same animal. In the Lumbricus genus, the reproductive system is located in segments 9 through 15.

Reproduction occurs throughout the year. In sexual intercourse, the worms put their ventral surfaces in contact, and can remain together thanks to the viscous substance that the clitellus produces. Certain special structures are in charge of penetrating the body of its partner and unloading the gametes.

Likewise, leeches are hermaphroditic, but are cross-fertilized during intercourse. Sperm is transferred by means of a penis or by a hypodermic impregnation - where a spermatophore penetrates the integument of another individual.

Breathing

Polychaetes exhibit a wide variety of adaptations to mediate respiratory processes. There are parapodia and gills in various species, which are responsible for mediating gas exchange. However, in certain species there is no type of body in charge of exchange.

In oligochaetes there are no specialized organs for respiration. For this reason, gaseous exchange occurs through the skin, which is a thin, moist surface that allows oxygen and carbon dioxide to pass through.

Most leeches breathe through their skin, although some have gills.

Feeding

Polychaetes that roam freely in aquatic environments are often predators or scavengers. In contrast, sedentary polychaetes feed on suspended particles or consume particles found in sediments.

Most oligochaetes are scavengers, and they feed on decomposing organic matter found in the soil. This can contain remains of leaves and vegetation or be organic matter from animals.

To avoid the high calcium concentrations typical of this type of diet, the oligochaetes have a series of glands, located along the esophagus, whose function is the excretion of excess ions, thus reducing the concentration of calcium in the blood. They also regulate the acid-base balance of body fluids.

Leeches are popularly considered parasitic organisms, however many are predators. It has specialized mouthparts for the intake of small invertebrates or for feeding on the blood of cold-blooded vertebrates.

Habitat and distribution

Annelids have a worldwide distribution, and some species are cosmopolitan. Polychaetes are mostly marine forms and although most are benthic, there are a number of pelagic forms that live in the open ocean.

Most oligochaetes and leeches are freshwater organisms or freshwater bodies. Some of the freshwater species tend to bury themselves under mud or sand, while others can plunge into vegetation.

References

1. Barnes, RD (1983). Invertebrate zoology. Interamerican.
2. Brusca, RC, & Brusca, GJ (2005). Invertebrates. McGraw-Hill.
3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
4. Irwin, MD, Stoner, JB, & Cobaugh, AM (Eds.). (2013). Zookeeping: an introduction to the science and technology. University of Chicago Press.
5. Marshall, AJ, & Williams, WD (1985). Zoology. Invertebrates (Vol. 1). I reversed.
6. Rousset, V., Pleijel, F., Rouse, GW, Erséus, C., & Siddall, ME (2007). A molecular phylogeny of annelids. Cladistics, 23 (1), 41-63.