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

References in periodicals archive ?
These findings suggest that the secondary ciliated band in the veliger is not homologous to the metatroch found in the trochophore larva of other spiralians, such as the annelid Polygordius lacteits (reportedly derived from 3c and 3d; Wollereck.
Temporal variation of annelids in the intertidal zone of beaches of the Sao Sebastiao Channel, southern Brazil.
Recent morphological and molecular phylogenies of annelids place the Oweniidae well within the Canalipalpata, as a member of either the Sabellida clade or a clade containing the Polygordiidae (Rouse and Fauchald, 1997, 1998; Rouse, 1999).
Second, the standard fixation protocols used for insect embryos include organic solvents that abrogate Hb-like staining in annelids (pers.
The traces Taenidium and Helminthoidichnites are usually thought to have been made by annelid worms.
Furthermore, they found that both brachiopods and annelids have two distinct Hox genes, called Post-1 and Post-2.
43) Thus these organisms would appear to be positioned somewhere on the evolutionary tree near the branching point of the mollusks with the annelids.
Leeches are annelids comprising the subclass Hirudinea.