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Related to Annelids: Arthropods, Platyhelminthes, Echinoderms



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.


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.


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This fact is an important neuroevolutionary advance, since we did not observe inhibitory synapses in planarians, neither in annelids (Palacios-Pru et al.
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Recorded arrests were much longer for the annelids. Activity of a long row of metatrochal cilia could be recorded in focus in cases where light passed through the larval body and an extensive part of the metatroch was near the focal plane.
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As modern arthropods and annelids served as appropriate analogs for the interpretation of this fossil, the researchers posit the animal in question could be the ancestor of either of the two groups.
Nevertheless, over the last decade, the training of young taxonomists has expanded investigation to other groups, such as polyclad flatworms (Quiroga, Bolanos, & Litvaitis, 2004), lophophorates (Florez-Romero, Montoya-Cadavid, Reyes-Forero, & Santodomingo, 2007), sipunculan annelids (Gomez, Ardila, & Sanjuan-Munoz, 2013), and nemerteans (Gonzalez-Cueto, Quiroga, & Norenburg, 2014).
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Phylum Species Annelids Neanthes kerguelensis Aphroditidea sp Serula narconensis Lumbrineridea sp Oligochaetes Chaetognaths Chaetognathe sp Crustaceans Euphausiids Decapods sp Copepods Coryceaus Cumacea sp Larvae of crustaceans Arthropods Serilis sp Pedunculata sp Natatolana sp Tanaids Tanaidacea sp Apseudomorpha sp Gastropod mollusks Cerithiopsidea sp Buccinidea sp Turridea sp Cancellariidea sp Nassariidae sp Epitoniidea sp Gasteropodea sp Enixotiophan sp Epoitonidea sp Fissurellidea sp Bivalve mollusks Crassatellidea sp Nuculana sp Bivalvia sp Crasostrea gasar Kidderia sp Euciroa sp Opistotranchia sp Cyaniidae sp Galeommatidea sp Limopsidea sp Cardiidea Gouldiopa sp Nematodes Priapulidea sp Sipunculidea sp Pennatulacea sp Platyhelminthes sp Broken eggs Fish?