Annelids


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Related to Annelids: Arthropods, Platyhelminthes, Echinoderms

Annelids

 

any worms of the subphylum Annelides, which consists of the most highly developed coelomate worms. Some zoologists classify the annelids as an independent phylum (Annelida). The body, ranging from several mm to 3 m (tropical earthworm), is bilaterally symmetrical and is divided internally by septa into segments, or somites. External rings, or annulations, correspond in number to the somites. The name of the worm is derived from the annulations. There may be as many as several hundred segments. Metamerism of the internal organs results from the body segmentation. On each segment there may be lateral appendages with setae; these are rudimental extremities, or parapods. The musculocutaneous sac consists of a thin cuticle, the peritoneal epithelium, and longitudinal and circular muscles.

The circulatory system is closed, with two main vessels, the dorsal and the ventral, which are connected by annular vessels. Respiration is cutaneous, and sometimes there are gill filaments. The excretory organs are metamerically paired nephridia. The central nervous system is composed of the brain, the subpharyn-geal ganglia, and the ventral nerve cord. Annelids are dioecious or hermaphroditic. Cleavage of the egg cells is spiral. The development of annelids is direct or larval (trochophore stage). In addition to sexual reproduction, there is asexual budding, or paratomy.

Annelids are divided into four classes. The most diverse class, Polychaeta, comprises over 6,000 species and is predominantly marine. The class Oligochaeta has approximately 3,000 species, which are found in the soil (earthworms) or in fresh water. There are approximately 300 species in the class Hirudinea (leeches). They are found in fresh water and in seas; some leeches live in damp soil among rotting leaves in tropical regions, and some species of Hirudinea are parasitic. The class Euchiuroidea consists of a few species of marine worms that burrow in the bottom of the sea. They lack body segmentation.

There are several theories of the origin of the annelid, the dominant one asserting it descended from the lower platyhelmin-thes, or flatworms. Polychaeta is considered to be the oldest class of annelids. As a result of the transfer to fresh water and terrestrial living, the oligochaetes descended from the polychaetes. The leeches, in turn, developed from the oligochaetes. Annelids have existed since the Middle Cambrian period.

REFERENCE

Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.
Rukovodstvo po zoologii, vol. 2. Moscow-Leningrad, 1940.
Handbuch der Zoologie, vol. 2, issue 2, fascs. 12–13. Leipzig, 1931.
Traité de zoologie, vol. 5. Paris, 1959.

P. V. USHAKOV

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New species of polychaetous annelids in the collections of the American Museum of Natural History from Porto Rico, Florida, Lower California and British Somaliland.
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The u-shape tube morphology observed here is similar to that typically produced by polychaete annelids of the family Spionidae (Blake & Evans 1973, Bromley & D'Alessandro 1983, Bromley 1994).
Parasites such as fungi, annelids and trematodes can disturb the extrapallial area by boring into or irritating the mantle epithelium (Sindermann 1990, Paillard et al.
Transport and mixing of gametes in three free-spawning polychaete annelids, Phragmatopoma californica (Feweks), Sabellaria cementarium (Moore) and Schizobranchia insignis (Bush).
Principal Groups Species Echinoderms Astropecten brasiliensis brasiliensis Muller & Troschel 1842 Arbacia dufresnii (Blainville 1825) Annelids Aphrodita longicomis Kinberg 1855 Bivalves Aequipecten tehuelchus (d'Orbigny 1846) Mytilus edulis platensis d'Orbigny 1846 Ostrea puelchana d'Orbigny 1841 Gastropods Adelomelon beckii (Broderip 1836) Crepidula argentina Simone, Pastorino & Penchaszadeh 2000 Crustaceans Libinia spinosa Milne Edwards 1834 Fishes Paralichtys patagonicus Jordan 1889 Percophis brasiliensis Quoy & Gaimard 1824 Squatina argentina (Marini 1930) Discopyge tschudii Heckel 1846
Sequencing is underway at JGI for a gastropod, the limpet Lottia scutum and two annelids, the leech Helobdella robusta and the polychaete worm Capitella capitata and at the NHGRI, for the fresh-water snail Biomphalaria glabrata.
The study found significant variability in domain organization with Taenia solium Dicer2 processing only one RNase III domain; the loss of the DEAD domain in Dicerl of mollusks, annelids, platyhelminths and most arthropods; and the absence of the PAZ domain in Dicer2 sequences of S.