Annelid

n. type of segmented worm

The annelids, collectively called Annelida (from Latin anellus "little ring"), are a large phylum of animals, comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches. They are found in most wet environments, and include many terrestrial, freshwater, and especially marine species (such as the polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimeter to over 3 meters (the seep tube worm Lamellibrachia luymesi). Annelids are triploblastic protostomes with a coelom, closed circulatory system and true segmentation. Their segmented bodies and coelom have given them evolutionary advantages over other worms. Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The coelom is divided into a sequence of compartments by walls called septa. In the most general forms each compartment corresponds to a triple segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. The closed circulatory system consists of networks of vessels containing blood with oxygen-carrying hemoglobin. Dorsal and ventral vessels are connected by segmental pairs of vessels. The dorsal vessel and five pairs of vessels that circle the esophagus of an earthworm are muscular and pump blood through the circulatory system. Tiny blood vessels are abundant in the earthworm's skin, which function as its respiratory organ. Each segment is marked externally by one or more rings, called annuli. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms,and in daria the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.
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Noun
1. worms with cylindrical bodies segmented both internally and externally
(synonym) annelid worm, segmented worm
(hypernym) worm
(hyponym) archiannelid
(member-holonym) Annelida, phylum Annelida
Adjective
1. relating to or belonging to or characteristic of any worms of the phylum Annelida
(synonym) annelidan
(pertainym) Annelida, phylum Annelida
(classification) zoology, zoological science

(a.)
Alt. of Annelidan
  

annelid /'ænilid/
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