Annelids make up an animal phylum of invertebrate organisms that are present in all types of habitats and are characterized by having a vermiform body composed of segments divided by rings. They include both earthworms and leeches, as well as a large number of marine worms.
In this Green Ecologist article we tell you what are annelids, their classification and examples, so if you want to know more about this edge of organisms, keep reading.
What are annelids and their characteristics
Annelids are eucelomatic animals that constitute a phylum (Annelida) of more than 10,000 species. They present bilateral symmetry and a vermiform morphology, which is commonly known as worm shape, with more than two cell layers, tissues and organs. Although most are aquatic organisms, they also occupy terrestrial environments.
Characteristics of annelids
The body of annelids is differentiated into an anterior region called the brothel (corresponding to the head), the log and a subsequent call the pygidium (the final part in which the anus is located). Annelids are characterized by having undergone an evolutionary process of metamerization, for which their body appears divided into metamers or segments arranged along the anteroposterior axis. The trunk of the annelids is the only region that has metamers, which are formed from the pygidium, so that the youngest segments are in front of it and the oldest are in the most anterior part. The trunk segments are separated by partitions or septa and each segment has a space inside that corresponds to the coelom, which is covered by a wall called the peritoneum. A series of morphological structures, such as blood vessels, musculature, and nephridia, are repeated in the trunk segments.
The body musculature of annelids is divided into circular muscles and longitudinal muscles. The contractions of the musculature are carried out by segments or groups of segments, along the anteroposterior axis, and propagate as a peristaltic wave through the body, getting the individual to advance, that is to say, that this is the mechanism that they use annelids to move around, crawling on the ground or swimming in water.
In the more evolved annelids, it is common for the prostomium to fuse with some of the trunk segments, constituting a secondary compound head. Then, the nervous system of the anterior part of the body also fuses and the ganglia of the nerve chain join to form a ganglionic complex. Annelids can present both sexual reproduction and hermaphroditism, have a closed circulatory system and lack respiratory organs, since they carry out gas exchange with the outside through the skin, that is, they are animals that have skin respiration. Here you can learn everything about Animals that breathe through skin.
Classification of annelids
The classification of annelids It occurs in three groups: polychaetes (Polychaeta), oligochaetes (Oligochaeta) and hirudíneos (Hirudinea).
Polychaetes (Polychaeta)
It is the most diverse and abundant group of annelids, with more than 6,000 species. These are marine annelids mostly, generally benthic and brightly colored. They differ between wanderers, who move freely through the substrate, and sedentary, who live in galleries that dig into the substrate. Wandering polychaetes generally have a flattened body, with well-developed heads and appendages on the trunk called parapods or podiums (one pair for each segment), which they use in swimming and from which quetas or silks emerge that cover the body surface and are remarkably visible. Polychaetes feed on algae and other nutrients that they find in the sand.
Most polychaetes are of separate sexes, although there are hermaphroditic specimens. They reproduce sexually and have an external copulation, so that the gametes are released into the water and fuse independently, giving rise to new organisms. In reproductive behavior, they usually develop a specialized segment that they eliminate during or after the release of gametes.
Oligochaetes (Oligochaeta)
The second most abundant group of annelids, with 3,100 species known. They live in diverse habitats, both terrestrial and freshwater, with some marine species. They feed on decomposing plant matter. They have an elongated body with a more rounded section, with a less developed head and less morphological diversity than polychaetes. They are characterized by lacking podiums and having few quetas that, in addition, are practically invisible to the naked eye. The prostomium appears underdeveloped, lacking sensory organs and appendages. The metastomy has a variable number of segments, with four pairs of lateral and four ventral ketae. A pair of nephridiopores per segment are also present in the metastomy, dorsal pores that connect the coelom with the external environment, and female and male pores in segments 14 and 15, respectively. The pygidium has a ring where the anus is located.
Oligochaetes are usually hermaphroditic animals and, upon reaching the adult stage, they develop a clitellus at the metastomium, which is an important organ in sexual reproduction. Oligochaetes are also capable of asexually reproducing through a fission process. Most oligochaetes are free-living, but there are also specimens within the parasitism life type. They lack sight and hearing, but they are able to detect light and vibrations and communicate through touch and taste.
Hirudineans (Hirudinea)
They know each other about 500 species of hirudíneos, too known as leeches. They are hermaphroditic organisms that inhabit freshwater environments, oceans, and moist soils. Some are predators that feed on worms, snails, fish, and invertebrates, and others are parasites. Certain Hirudines are blood-sucking and others are not. Its flattened and elongated body is dark in color and has suction cups at each end; the prostomium and the first five body segments are fused to form a venous that contains the mouth, while the pygidium is fused to the last seven segments of the back, giving rise to another suction cup that houses the anus. The body of polychaetes has a fixed number of segments (33), although the number of rings per segment varies between species. They are also characterized by not presenting quetas or podiums.
In the anterior area of the body, the genital orifices are observed, each individual has a female and a male genital pore, and in addition, upon reaching sexual maturity, they develop a clitellum. Some species have eyes and many have sharp teeth or even jaws. They are deaf and have underdeveloped vision, so they rely on touch and the sense of taste to communicate.
Examples of annelids
Once explained what they are and their characteristics, some simple ones are cited examples of annelids:
- Oligochaetes: common earthwormLumbricus terrestris) Y Tubifex tubifex.
- Polychaetes: serpentine tubewormSerpula vermicularis) Y Arenicola marina.
- Hirudíneos: He hasemadipsa picta and medicinal leechHirudo medicinalis).
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