Oligochaeta

Oligochaeta Temporal range: Triassic – recent PreꞒ Ꞓ O S D C P T J K Pg N
Earthworm (Lumbricus terrestris)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Annelida
Clade:	Pleistoannelida
Clade:	Sedentaria
Class:	Clitellata
Subclass:	Oligochaeta
Orders
Haplotaxida Lumbriculida Moniligastrida

Oligochaeta (

Terrestrial oligochaetes are commonly known as earthworms and burrow into the soil. The four main families with large numbers of species are Glossoscolecidae, Lumbricidae, Megascolecidae and Moniligastridae. Earthworms are found in all parts of the world except for deserts. They have a requirement for moist surroundings and the larger species create burrows that may go down several metres (yards) while young individuals and smaller species are restricted to the top few centimetres of soil. The largest numbers are found in humus-rich soils and acid soils. A few species are found in trees, among damp moss and in the debris that accumulates in leaf axils and crevices; some others make their homes in the rosettes of bromeliads.^[3]

The majority of aquatic oligochaetes are small, slender worms, whose organs can be seen through the transparent body wall. They burrow into the sediment or live among the vegetation mostly in shallow, freshwater environments. Some are transitional between terrestrial and aquatic habitats, inhabiting swamps, mud or the borders of water bodies. About two hundred species are marine, mostly in the families Enchytraeidae and Naididae; these are found largely in the tidal and shallow subtidal zones, but a few are found at abyssal depths.^[3]

Anatomy

The first segment, or prostomium, of oligochaetes is usually a smooth lobe or cone without sensory organs, although it is sometimes extended to form a tentacle. The remaining segments have no appendages, but they do have a small number of bristles, or chaetae. These tend to be longer in aquatic forms than in the burrowing earthworms, and can have a variety of shapes.

Each segment has four bundles of chaetae, with two on the underside, and the others on the sides. The bundles can contain one to 25 chaetae, and include muscles to pull them in and out of the body. This enables the worm to gain a grip on the soil or mud as it burrows into the substrate. When burrowing, the body moves peristaltically, alternately contracting and stretching to push itself forward.

A number of segments in the forward part of the body are modified by the presence of numerous secretory glands. Together, they form the clitellum, which is important in reproduction.^[4]

Internal anatomy

Most oligochaetes are detritus feeders, although some genera are predaceous, such as Agriodrilus and Phagodrilus. The digestive tract is essentially a tube running the length of the body, but has a powerful muscular pharynx immediately behind the mouth cavity. In many species, the pharynx simply helps the worm suck in food, but in many aquatic species, it can be turned inside out and placed over food like a suction cup before being pulled back in.

The remainder of the digestive tract may include a crop for storage of food, and a gizzard for grinding it up, although these are not present in all species. The

Oligochaetes occur in every continent in the world occupying terrestrial, freshwater and marine habitats. Of the 1700 known aquatic species, about 600 are marine and 100 inhabit groundwater. Aquatic oligochaetes occur in most groups, with the Naididae being the most speciose.^[5]

Locomotion

Movement and burrowing of earthworms is performed by peristalsis, with the alternation of contraction and relaxation of the circular and longitudinal muscles. To move forward, the anterior portion of the worm is extended forward by the contraction of the circular muscles, while the portion just behind this is made shorter and fatter by the contraction of longitudinal muscles. Next the anterior circular muscles relax, and a wave of circular contraction moves backwards along the worm.^[6] At the same time, the cheatae expand to grip the ground as the body shortens and are retracted as it lengthens. The steps are typically 2 to 3 cm (0.8 to 1.2 in) long and the worm moves at the rate of seven to ten steps per minute. The worm is able to reverse its direction of travel with the tail leading. Aquatic species use a similar means of locomotion to work their way through sediment and massed vegetation, but the tiny Aeolosomatids swim by means of the cilia on their prostomia.^[3]

Burrowing is performed by forcing the front end of the worm into a crevice and widening the gap by body expansion. Large quantities of soil are swallowed in the process. This is mixed with mucus as it passes through the gut, being used to plaster the tunnel walls, forming a lining. Excess material is extruded on the ground surface, forming a faecal casting. The burrow may have two entrances and several vertical and horizontal tunnels.^[3]

Reproduction

Whereas in general, polychaetes are marine and have separate sexes, external sperm transfer and external fertilisation, oligochaetes live on land or in fresh water, are hermaphrodites, have no external sperm transfer and fertilisation takes place in the clitellum or cocoon. However there are exceptions to this, with some polychaetes inhabiting non-marine environments and a few species of oligochaetes being marine.^[6] Development of the offspring also differs between the two subclasses. The eggs of polychaetes are deposited in the sea where they develop into trochophore larvae that disperse as part of the plankton, while the yolky eggs of oligochaetes do not have a larval stage and develop directly into juvenile worms in the cocoon.^[6]

Reproduction among oligochaetes is mainly by sexual means but clonal reproduction is common in some genera, especially among aquatic species. Members of the Naididae reproduce asexually, primarily by paratomy, in which the body breaks into two pieces after the "pregeneration" of certain anterior structures by the posterior portion. Other species undergo fragmentation, in which the worm breaks into several pieces, each of which develops into a new worm. Parthenogenesis also occurs in some species.^[5]

Evolution and taxonomy

With their soft bodies, earthworms do not fossilize well, though they may form trace fossils.^[7] The name Protoscolex was given to a genus of segmented worms without bristles found in the Upper Ordovician of Kentucky, United States. Another species placed in the same genus was found in Herefordshire, England, but it is unclear whether these worms are in fact oligochaetes. Stephenson postulated in 1930 that the common ancestor of oligochaetes came from the primitive aquatic family Lumbriculidae. The more advanced families such as Glossoscolecidae, Hormogastridae, Lumbricidae and Microchaetidae may have evolved later than the other families. Because of its ability to colonise new areas and become dominant, the Lumbricidae has followed humans round the world and displaced many native species of earthworm.^[8]

An early but now outdated classification system was to divide the oligochaetes into "Megadrili", the larger terrestrial species, and "Microdili", the smaller, mostly aquatic ones.^[8]

Families

References