Latin Oligochaeta; from Dr. Greek ὀλίγος 'little' and Dr. Greek χαίτη 'hair'.

Oligochaeta - small bristle worms. Class roundworms. Characterized by the absence of paranodia, tentacles and an insignificant number of bristles located on the sides of each segment, except for the mouth. Sensory organs are absent. Hermaphrodites. Are part of the zoobenthos, a natural food base for fish. Dwell in the silt sediments of rivers, lakes, ponds, reservoirs. Contributes to self-purification of polluted water bodies.

Oligochaeta are mainly terrestrial and freshwater annelids; some species have transitioned secondarily to marine environments, especially estuaries and interstitials. Freshwater Oligochaeta, possessing generally small body sizes, inhabit clay and muddy sediments and among aquatic vegetation. Marine Oligochaeta are mostly tiny interstitial animals found at a wide range of depths, from the littoral to deep water.

The body of Oligochaeta is strongly elongated, cylindrical. Body length ranges from fractions of a millimeter to 2.5 m (some earthworms). There is a secondary body cavity, the integument. Segmentation of the body is well expressed inside and outside. The number of segments ranges from 5-7 to 600. Each segment has coelomic sacs that close on the ventral and dorsal sides. These sacs are repeated in each segment and seem to be suspended from the mesentery, or mesentery. Two neighboring coelomic sacs are separated by septa - dissepiments. Parapodia are absent. Covers contain a large number of cutaneous glands that secrete mucus. Each segment of the body contains several pairs of bristles assembled in bundles. The number of bristles in a bundle varies from one to 25. The head is weakly expressed. In most species, respiration is cutaneous, specialized gills are only in a few Oligochaeta. Some species are characterized by so-called "breathing movements" - oscillations of the body in the flow of water.

The digestive organs are usually large and adapted to pass large masses of soil and bottom soil, which most worms feed on. There are three divisions in the digestive system: anterior, middle, and posterior. The anterior includes the mouth cavity, muscular pharynx, esophagus, goiter, and muscular stomach. The ducts of three pairs of calcareous glands open into the esophageal cavity of earthworms, whose secretions neutralize humic acids contained in the food. The midgut has a fold, the typhlosol, located dorsally (i.e., on the dorsal side) that increases the suction surface.

The nervous system is of the nodal (ganglionary) type. There are paired supraglottic ganglia connected with the abdominal nerve chain by connectives, and two neighboring nerve nodes of the abdominal nerve chain in a segment are connected by commissures. In turn, the paired supraglottic ganglia (called the medulla) are divided into three parts: protocerebrum, mesocerebrum, deutocerebrum. The sensory organs are poorly developed. Eyes and tentacles in most forms are absent. There are sensory bristles, olfactory fossae and statocytes.

The excretory system is represented by metanephridia of the coelomoduct type. They begin as funnels (nephrostomas) in the coelom, and open outward in the next segment nephridiopores, and in the tubules of metanephridia there is a thickening of metabolic products, and the fluid is again ejected in the whole (this is an adaptation to life on land). In some species, just before the nephridiopore, the tubule dilates to form a urinary vesicle. The nephridiopores are usually located on the ventro-lateral surface of each segment. Typically, two metanephridial tubules are present in each segment. The only exceptions are the most anterior and most posterior body segments.

Oligochaeta are characterized by synchronous hermaphroditism, carrying a small number of large, yolk-rich eggs in a protective and nourishing structure, the cocoon secreted by the girdle, and a highly reduced number of gonads. Well-developed gonads are localized in a small number of sexual segments in the anterior half of the worm. The female sexual segment(s) are always located subsequent to the male ones. Most aquatic forms have one male segment followed by one female segment, whereas species belonging to terrestrial taxa often have two male segments. The position of the sexual segments varies among members of different taxa. The paired ovaries and testes are located on the anterior septa of the sexual segment. They are attached to its posterior surface and protrude into the coelomic cavity. Maturing gametes are released from the gonads in the early stages of gametogenesis and fall into specialized coelomic sacs, which are respectively called seminal sacs (for sperm) or oviductal sacs (for developing eggs). In these sacs the formation of gametes is completed. Each sexual segment is provided with two gonoducts; the seminal ducts for the excretion of sperm or the oviducts through which the eggs are excreted. The gonoducts extend backward through one or more segments before opening on the ventral surface of the body.

The circulatory system is closed. It is represented by dorsal and abdominal vessels connected by annular bridges. Blood movement is provided by the contraction of the dorsal and annular vessels. In earthworms, several specific features are found, which include the presence of capillaries in the thickness of the coverings and the presence of hearts, which in many Oligochaeta functionally complement the work of the contractile dorsal blood vessel. The hearts are dilated muscular regions of the perineal vessels linking the ventral and dorsal longitudinal vessels. The number of hearts varies. Lumbricus has five pairs of hearts with rings encircling the esophagus. Tubifex has only one pair of peri-intestinal hearts. The hearts are equipped with valves, which are folds of their walls.

Most aquatic and terrestrial Oligochaeta are "scavengers", feeding on dead organic matter, primarily of plant origin. Earthworms feed on decomposing surface soil organic matter and may drag fallen leaves into their burrows. Being detritophagous, they ingest organic material along with the ingested soil. Fine detritus, algae and microorganisms are important food sources for the small freshwater forms. Frequently encountered small Aeolosoma collect detritus with the help of the prostomium. While feeding, the worm presses the ventral, cilia-covered surface of the prostomium against the substrate, after which the central part of this ciliated field is lifted by muscle contraction. A rarefaction is created in the formed inverted "cup", which detaches small particles of detritus from the substrate. Actively working cilia of Prostomium drive them into the mouth. Representatives of the genus Chaetogaster - small carnivorous Oligochaeta - capture food (amoebae, infusoria, rotifers and larvae of trematodes), using for this purpose sucking throat. They also often parasitize freshwater mollusks.

Oligochaeta are hermaphrodites. They reproduce by cross fertilization. Copulation begins when two worms are placed towards each other and come into contact with each other, touching the ventral surfaces of the anterior parts of the body. In most oligochaetes, the male sexual orifices of one individual are located directly opposite the seminiferous receptors of the other. The mucus secreted by the girdles envelops the worms, helping them to stay in this position. In addition, adhesion is further ensured by specialized sexual bristles located in the region of the male genital opening or the seminal receptors. This is followed by mutual exchange of seminal fluid. The mated worms then disperse. Oligochaetes are characterized by direct development and consequently have rather yolk-rich eggs. The eggs of aquatic forms tend to contain more yolk than those of terrestrial species. Development takes place within a cocoon; it lasts from eight days to several months, after which a juvenile individual emerges from the cocoon. Vegetative (sexless) reproduction is widespread among aquatic oligochaetes, especially Aeolosomatidae, Naididae and Lumbriculidae.