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annelid, phylum name Annelida, also referred to as segmented worm, any kind of member the a phylum the invertebrate animals that are characterized by the possession of a human body cavity (or coelom), movable bristles (or setae), and also a body separated into segments by transverse rings, or annulations, indigenous which lock take your name. The coelom is reduced in leeches, and setae are lacking a few devoted forms, consisting of leeches. A significant invertebrate phylum that the pet kingdom, the annelids number an ext than 9,000 types distributed amongst three classes: the maritime worms (Polychaeta), which are split into free-moving and sedentary, or tube-dwelling, forms; the earthworms (Oligochaeta); and also the leeches (Hirudinea).

General features

Distribution and abundance

Annelids are found worldwide in all types of habitats, especially oceanic waters, new waters, and damp soils. Most polychaetes live in the ocean, where they one of two people float, burrow, float on the bottom, or live in tubes lock construct; your colours selection from brilliant to dull, and also some varieties can produce light. The feather duster (Manayunkia speciosa) inhabits the great Lakes and also some rivers the the unified States. The polychaetes include an ext than 6,000 known species, which are about evenly divided in between free-moving and also tube-dwelling forms. The oligochaetes number around 3,250 known species. Oligochaetes, consisting of earthworms, burrow into soil; certain tiny oligochaetes are discovered in fresh water, and a few are marine, usually inhabiting estuarial or various other shallow waters. Leeches, i m sorry number around 300 species, inhabit freshwater or humid environments and are carnivorous or parasitic on other organisms—e.g., all naval leeches are parasitic ~ above fish.

Size variety and diversity that structure

The size of annelids varies from a fraction of an customs to more than six metres (about 20 feet). The width may exceed 2.5 centimetres (about one inch) in the contracted state. Free-moving polychaetes and also earthworms incorporate the biggest species. Leeches achieve lengths of about 0.4 metre in the contract state.

Polychaetes

The human body of free-moving polychaetes (see figure) consists of a head, or prostomium, which might bear two or more eyes; a preoral segment, with such appendages as antennae, tentacles, and palpi (fleshy sensory projections); a tribe divisible into distinct segments; and also a tail, or pygidium, which might bear anal cirri (fleshy projections) or plaques and also a terminal anus. Each body segment following the 2nd segment (peristome) usually has actually paired parapodia; i.e., fleshy, lateral outgrowths provided in feeding, locomotion, or breathing. The parapodia, normally prominent in free-moving polychaetes, bear bundles of setae, which deserve to be extended, and aciculae (needlelike structures), i m sorry are used for support.


The top of sedentary polychaetes (see figure) might be distinct or indistinct. Develops with a distinct head usually lack head appendages. Branchiae, or gills, which offer for respiration and also as food-gathering organs, space well-developed in numerous of the tube-dwelling forms. Some have tentacles in ~ the anterior (front) end, and gills arise native the dorsal (upper) surface ar of a couple of anterior segments. In these species food is gathered through the tentacles and also respiration is confined to the gills. The rest of the human body is separated into thoracic and abdominal regions. Parapodia, if present, are generally straightforward lobes; commonly the setae project directly from the body wall. Many sedentary polychaetes build tubes make from a problem secreted native cells the constitute the epidermis, or skin. Tubes might consist the calcium carbonate, parchment, or mucus, to which sediment adheres. The anus is at the posterior tip. Pipe dwellers generally have an exterior fecal groove along which fecal material passes forward. Eyes room occasionally current on gills, along the sides of the body, or top top the pygidium in sedentary forms that carry out not live in tubes.

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Oligochaetes

The human body of oligochaetes is uniformly segmented and also has conspicuous segmental lines. The prostomium is commonly a basic lobe overhanging the mouth and lacking appendages. The microscopically little eyes are scattered over the body. The clitellum, a saddle-shaped thickening the the body wall, is existing at sexual maturity. The anus is in ~ the posterior tip. Setae normally arise from the ventral (lower) surface of the body.