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Oligochaeta (singular Oligochaete, pronounced /ˈɒlɨɡɵkiːt/) is a subclass in the biological phylum Annelida and includes various earthworms. Specifically, it contains the terrestrial megadrile earthworms (some of which are semi- or fully aquatic), and freshwater or semi-terrestrial microdrile forms including the tubificids, pot worms and ice worms (Enchytraeidae), blackworms (Lumbriculidae) and several interstitial marine worms. With around 10,000 known species the Oligochaeta make up about one half of the phylum. These worms usually have few setae (chaetae) or "bristles", and lack parapodia, unlike polychaeta.
[edit] Common characteristicsOligochaetes are well-segmented worms and most have a spacious body cavity (coelom) that is used as a hydroskeleton. They range in length from less than 0.5 millimetres (0.020 in) up to 2 to 3 metres (6.6 to 9.8 ft), in the case of species such as the giant Gippsland earthworm. The first segment, or prostomium, of oligochaetes is usually a smooth lobe or cone no 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 setae. 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 setae, with two igloos on the underside, and the others on the sides. The bundles can contain anything from one to twenty-five setae, 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.[1] [edit] Internal anatomyMost 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 oesophagus includes "calciferous glands" that maintain calcium balance by excreting indigestible calcium carbonate into the gut. A number of yellowish "chloragogen cells" surround the intestine and the dorsal blood vessel, forming a tissue that functions in a similar fashion to the vertebrate liver. Some of these cells also float freely in the body cavity, where they are referred to as "eleocytes".[1] Most oligochaetes have no gills or similar structures, and simply breathe through their moist skin. The few exceptions generally have simple, filamentous gills. Excretion is through small ducts known as metanephridia. Terrestrial oligochaetes secrete urea, but the aquatic forms typically secrete ammonia, which dissolves rapidly into the water.[1] The vascular system consists of two main vessels connected by lateral vessels in each segment. Blood is carried forward in the dorsal vessel (in the upper part of the body) and back through the ventral vessel (underneath), before passing into a sinus surrounding the intestine. Some of the smaller vessels are muscular, effectively forming hearts; from one to five pairs of such hearts is typical. The blood of oligochaetes contains haemoglobin in all but the smallest of species, which have no need of respiratory pigments.[1] The nervous system consists of two ventral nerve cords, which are usually fused into a single structure, and three to four pairs of smaller nerves per body segment. Only a few aquatic oligochaetes have eyes, and even then they are only simply ocelli. Nonetheless, their skin has several individual photoreceptors, allowing the worm to sense the presence of light, and burrow away from it. Oligochaetes can taste their surroundings using chemoreceptors located in tubercles across their body, and their skin is also supplied with numerous free nerve endings that presumably contribute to their sense of touch.[1] [edit] Life cycleEarthworms are hermaphrodites, which means that each animal has both male and female reproductive organs. They have external fertilization (except for some members of the African family Eudrilidae), but copulate and store sperm in a receptacle called a spermatheca. Like leeches, they have a clitellum which secretes a "cocoon" or capsule into which both eggs and sperm are deposited and acts as an incubator for the embryonic worms. The cocoon is deposited in the soil. On hatching, the young worms resemble small adults and grow continually until they reach maturity. They lack a trochophore larval stage. [edit] HabitatNative earthworm species are often eradicated from natural areas as people clear native vegetation and introduced species become more dominant in these disturbed habitats. Introduced earthworms are most common in disturbed environments such as suburban gardens and farmland paddocks. [edit] FamiliesThe following list of Oligochaeta families follows ICZN convention so that family-group name (ending in -idae) is followed by authorship and date. 
[edit] References[edit] Bibliography
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