CLITELADOS: CHARACTERISTICS, CLASSIFICATION, REPRODUCTION - BIOLOGY - 2025

Clitellata is one of the classes in which the phylum annelida is divided. It is made up of oligochaetes such as the earthworm and Hirudineans such as the leech. It was described by the French naturalist Jean Baptiste Lamarck in 1809.

Among its most outstanding characteristics is the presence of a structure called the clitellus, which performs reproductive functions. Its name derives from the presence of that organ. The common name of the members of this class is, in fact, "clitelados."

The earthworm is a representative species of the clitelados. Source: Michael Linnenbach

Regarding the classification, there is a discrepancy between the specialists. Some state that the class is divided into three subclasses: oligochaeta, hirudinea, and branchiobdellae. However, most suggest that the Branchiobellidae are included within the Hirudineans.

Taxonomy

The taxonomic classification of the clitelados is the following:

- Domain: Eukarya.

- Animalia Kingdom.

- Phylum: Annelida.

- Class: Clitellata.

characteristics

Clitelates are organisms that are made up of eukaryotic cells, which implies that their genetic material (DNA) is enclosed within the nucleus, delimited by a nuclear membrane. Likewise, their cells have reached a certain level of specialization, which is why they are considered multicellular organisms.

Likewise, clitelates can be found in terrestrial and wet freshwater habitats. They can also be found in arid places such as deserts and also in tropical habitats, inhabiting between vegetative strata and litter.

These organisms are hermaphroditic, so they contain both female and male reproductive structures and reproduce mainly through sexual mechanisms that involve internal fertilization.

Morphology

As members of the phylum annelida, the clitelados are characterized by having a segmented body. Each segment is known as a metamer. They can also have a cylindrical body (oligochaetes) or flattened (hirudineos).

-External anatomy

Importantly, clitelates have a specific number of metamers. Of course, depending on the class of clitelates, the number of metamers will be different. In the case of oligochaetes, the number of segments is between 100 and 160, while in Hirudineans, the number of segments ranges from 15 to 34, depending on the different species.

In general, the body of the clitelates is divided into three regions: head, trunk and pygidium.

Head

On the head is a structure known as a prostomy. Here too is the mouth. In the case of the Hirudineans, the mouth is a suction cup through which they can attach themselves to the environment and suck the food (for example blood).

Trunk

It makes up most of the animal's body. In some of the clitelates, such as oligochaetes, certain short extensions emerge from each segment, called quetas. They actively participate in the locomotion and movement of the animal. In the case of the Hirudineans, these extensions are non-existent.

On both sides of the body you can see pores, which are known by the name of nephridiopores, and which perform functions in the excretory system.

Likewise, their reproductive systems, both male and female, flow into pores that are located in specific segments of the animal. In the case of oligochaetes, the male pores are in segment 15, while the female pores are in segment 14. In the case of Hirudineans, the female pore opens in segment 11 and the male pore in one of the previous segments.

The body of the clitelates presents a thickening of the epithelium that is called the clitellum. This structure fulfills functions during the reproductive process of the animal, secreting a mucus that facilitates coupling for copulation, as well as forming the cocoon in which fertilization and subsequent development of the eggs take place.

Pygidium

It is the last segment of the animal's body. Here is the anus and, in the case of the Hirudineans, there is also the posterior sucker.

-Internal anatomy

Body wall

The body of the clitelados is made up of several layers:

- Cuticle: it is the outermost layer and is secreted by the epidermis of the animal. It is thin and has protective purposes.

- Epidermis: it is located between the cuticle and the basement membrane. It contains specialized cells such as mucosal cells, albuminoid cells, and basal cells.

- Basement membrane: thin layer that separates the epidermis from the muscular layers.

- Circular and longitudinal muscles: layers containing muscle fibers that, when contracting and relaxing, contribute, among other things, to the animal's movement process.

Beneath the musculature is a cavity known as the coelom.

Coelom

It is the internal cavity of the animal's body. Depending on the class, the coelom has different characteristics. In the case of the Hirudineans, it is filled with a kind of compact tissue called botryoidal tissue, which has various functions, such as producing hemoglobin and excretory function.

On the other hand, in oligochaetes, the coelom is a cavity that is filled with fluid, in which some organs such as the intestine are suspended. In them the coelom is segmented by partitions.

Digestive system

The digestive system is made up of a tube divided into different parts, each one specialized in a specific function. They present an oral cavity, which continues with the pharynx.

This is followed by the esophagus and stomach. It is important to note that in oligochaetes the stomach is divided into crop and gizzard, while in Hirudineans it only consists of the crop.

Immediately after the stomach is the intestine, followed by the rectum and finally the anus.

In spite of the rudimentary that clitelates may seem, their digestive system is highly specialized and, if they cannot fulfill any function, such as the digestion of proteins, they have bacteria that inhabit the digestive tract and carry out the work. for them.

Internal anatomy of an oligochaete. (1) Brain ganglion. (2) Pharynx. (3) Heart. (5) Seminal vesicles. (6) Crop. (7) Gizzard. (8) Intestine. Source: LenaWild

Excretory system

It is made up of metanephridiums, which have two ends, the nephrostoma, which opens towards the coelomic cavity, and the nephridiopore, which flows directly outside.

Nervous system

Despite being very little evolved organisms, clitelates have a nervous system made up of specialized structures.

In general, the clitoral nervous system is made up of two ventral nerve chains, cerebroid ganglia, a periosophageal collar, and metameric ganglia. They also have cells specialized in the secretion of neurohormones that regulate various functions of the body.

Regarding the sensory receptors, the clitelates present photoreceptors that allow them to receive light stimuli, chemoreceptors that allow them to perceive chemical signals of different kinds, as well as free endings with tactile function.

Reproductive system

Clitelates are hermaphroditic organisms, that is, they have both female and male reproductive organs.

The male reproductive system is made up of pairs of testicles and the seminal vesicle. Likewise, they can present efferent and deferent channels. It ends in a male pore that, depending on the species, opens in a specific segment of the animal's body.

The female reproductive system is made up of ovaries, an oviscus where the eggs are stored, and an oviduct that emerges from each oviscus. They also have a female pore that opens in a specific segment, depending on the species.

Feeding

Diet is highly dependent on subclass. In this sense, the nutritional characteristics of the oligochaetes are totally different from those of the Hirudineans.

In the case of oligochaetes, there will be organisms that are predators and eat small animals and others that feed on organic and plant detritus, as well as the remains of earth. Generally, the waste products of its digestion are enriched matter that contributes to fertilizing the soil.

On the other hand, some species of Hirudineans are widely known as hematophages, that is, they feed on blood. Others feed on small prey. In the latter, the prey is ingested whole and is digested with the help of certain bacteria that secrete endopectidase enzymes (they degrade proteins).

In the case of hematophogous hirudineans, they fixate on the host through their oral suction cup and begin to suck blood. The host is unaware because the hirudinea secretes an anesthetic.

These animals are capable of absorbing a large amount of blood, several times their body size. Likewise, the bacteria present in your digestive tract contribute to digestion.

Reproduction

Clitelados reproduce sexually. This type of reproduction involves the union of gametes.

Even though these organisms are hermaphrodites, they mate with each other. For copulation, two specimens line up in opposite directions, that is, with the head facing the other's tail. In this position, the male and female pores come into contact and both animals unite, thanks, in part, to mucous secretions produced by the clitellus.

Depending on the species, fertilization can occur within the ovisac (hirudineos) or in the cocoon formed by the clitellus (oligochaetes). Regardless of how it occurs, the eggs resulting from this fertilization are stored in a cocoon. In the case of oligochaetes, there may be up to 20 eggs per cocoon, while in Hirudineans, there is only one egg per cocoon.

The development of these organisms is direct, that is, there is no larval stage nor do they undergo metamorphosis. Individuals with the same characteristics as the adult specimens emerge from the cocoons.

Classification

Clitelates are a large group of organisms that are classified into two subclasses: oligochaeta (worms) and hirudinea (leeches).

Oligochaeta

It is a subclass of the Clitellata class that encompasses more than 3,000 species. The most representative species of this subclass is the earthworm. They are characterized by having a segmented tubular body with variable lengths.

Likewise, they are located in both terrestrial and freshwater habitats. Most species are of great help in ecosystems, since the waste products of their digestion are widely used as compost and soil fertilizer.

Hirudinea

They are very flexible organisms that, like oligochaetes, have a clitellum that performs functions within the reproductive process. The body of the Hirudineans is flat, and they are of variable length.

Hirudinea specimen. Source: GlebK

Among the annelids, the organisms of the hirudine subclass are considered among the most evolved. Some of the members of this group, such as the Hirudo medicinalis, are used in the medical field to carry out bleeding processes, thanks to their blood-sucking nutrition.

References

1. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition
3. Hernández, L., Ojeda, A. and López, D. (2012). Bioecological characteristics in populations of earthworms (Oligochaeta: Glossoscolecidae) of a natural and a protected savanna in the central plains of Venezuela. Journal of Tropical Biology. 60 (3).
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
5. Martinson, S. (2016). Exploring the species boundaries interrestrial clitellates (Annelida: clitellata). University of Gothenburg, Sweden.
6. Tarmo, T. and Martin, P. (2015). Clitellata: Oligochaeta. Chapter of Book: Thorp and Covich's freshwater invertebrates. Academic press. 4 ^th