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annelid, phylum surname Annelida, also referred to as segmented worm, any type of member of a phylum that invertebrate pets that are defined by the possession that a human body cavity (or coelom), movable bristles (or setae), and also a body divided into segment by transverse rings, or annulations, indigenous which castle take your name. The coelom is diminished in leeches, and setae are doing not have a few specialized forms, consisting of leeches. A significant invertebrate phylum of the pet kingdom, the annelids number much more than 9,000 species distributed amongst three classes: the maritime worms (Polychaeta), which are divided into free-moving and also sedentary, or tube-dwelling, forms; the earthworms (Oligochaeta); and also the leeches (Hirudinea).

General features

Distribution and also abundance

Annelids room found an international in all varieties of habitats, specifically oceanic waters, new waters, and also damp soils. Most polychaetes live in the ocean, whereby they either float, burrow, float on the bottom, or live in tubes castle construct; your colours range from brilliant to dull, and some varieties can develop light. The feather duster (Manayunkia speciosa) inhabits the great Lakes and also some rivers the the joined States. The polychaetes include more than 6,000 recognized species, which are about evenly divided between free-moving and also tube-dwelling forms. The oligochaetes number around 3,250 well-known species. Oligochaetes, consisting of earthworms, burrow into soil; certain little oligochaetes are found in new water, and a couple of are marine, normally inhabiting estuarial or other shallow waters. Leeches, which number around 300 species, inhabit freshwater or humid environments and are carnivorous or parasitic on various other organisms—e.g., all naval leeches are parasitic on fish.

Size range and diversity the structure

The length of annelids different from a fraction of an customs to more than six metres (about 20 feet). The width might exceed 2.5 centimetres (about one inch) in the contracted state. Free-moving polychaetes and also earthworms encompass the largest species. Leeches attain lengths of about 0.4 metre in the contracted state.

Polychaetes

The body of free-moving polychaetes (see figure) consists of a head, or prostomium, which may bear 2 or more eyes; a preoral segment, v such appendages together antennae, tentacles, and palpi (fleshy sensory projections); a tribe divisible into unique segments; and a tail, or pygidium, which may 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 supplied in feeding, locomotion, or breathing. The parapodia, usually prominent in free-moving polychaetes, bear majority of setae, which deserve to be extended, and aciculae (needlelike structures), which are supplied for support.


The heads of sedentary polychaetes (see figure) might be unique or indistinct. Creates with a distinctive head generally lack head appendages. Branchiae, or gills, which offer for respiration and as food-gathering organs, room well-developed in countless 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 by the tentacles and also respiration is confined come the gills. The remainder of the body is divided into thoracic and abdominal muscle regions. Parapodia, if present, space generally simple lobes; commonly the setae project directly from the human body wall. Plenty of sedentary polychaetes build tubes do from a substance secreted from cells that constitute the epidermis, or skin. Tubes may consist of calcium carbonate, parchment, or mucus, come which sediment adheres. The anus is at the posterior tip. Tube dwellers typically have an outside fecal groove along which fecal material passes forward. Eyes are occasionally current on gills, along the political parties of the body, or ~ above the pygidium in sedentary creates 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 usually a straightforward lobe overhanging the mouth and lacking appendages. The microscopically tiny eyes room scattered over the body. The clitellum, a saddle-shaped thickening of the human body wall, is present at sexual maturity. The anus is at the posterior tip. Setae typically arise native the ventral (lower) surface ar of the body.