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Phylum Annelida



Polychaete worm, class Polychaeta, phylum Annelida, modified from Bianco, 1904.
Annelid worms are primarily inhabitants of the marine environment, although there are many freshwater and terrestrial forms as well. All Annelids are segmented worms, with about 15,000 living species presently described. They range in size from small meiofaunal species (less than 0.5 mm long) to species that reach over 3 meters in length (giant terrestrial earthworms in the southern hemisphere ).

Classes of the phylum Annelida :
  • Class Polychaeta (mostly marine forms)
  • Class Oligochaeta (earthworms, tubificids, and others)
  • Class Hirudinea (leeches)
CLASSES OF ANNELIDS
Class Polychaeta



Serpulid Polychaete (out of its tube), Order Sabellida , from Van Cleave 1924.
Polychaete worms are known by a variety of common names (sand-, pile-, featherduster- , and christmas tree- worms, among many others). There is a wide diversity of body forms and lifestyles within the polychaetes . There are two basic lifestyles of Polychaetes , -the errant forms (swimming, crawling , burrowing), and those that create tubes or burrows and live within them. Many of the errant forms (such as the one pictured at the top of the page) are usually benthic predators that crawl through mud, sand, seaweed, or under stones in search of prey, which they may sieze with powerful jaws bearing chitinous teeth. Some polychaetes with strong jaws are omnivorous or herbivorous , using their jaws and teeth to tear off pieces of algae . Some forms burrow through soft sediments as direct deposit feeders, ingesting sediments and extracting organic material as it passes through their gut (much as an earthworm does). Tube dwelling forms make a variety of tubes or burrows. Some create U-shaped burrows through which they pump water and filter out food particles. Some species build tubes out of sand grains, parchment like material, or hard calcium carbonate tubes cemented to rocks. These sessile tube dwelling forms use tentacles of various kinds to either filter particles out of the water column, or to pick up organic particles that have settled to the bottom. Others species are symbiotic or parasitic , living in bivalves , corals , and a variety of other organisms. There are pelagic polychaetes (often somewhat transparent), as well as minute interstitial forms that live in the speces between grains of sand.
Class Oligochaeta
The approximately 3,100 species of Oligochaetes are mostly marine and terrestrial, with about 200 marine species presently described. The most familiar forms are the earthworms , which are terrestrial (but require relatively moist soil). The Oligochaetes probably evolved from the ancestral annelid line independently from the Polychaetes and Hirudinids.
Class Hirudinida (leeches)
The approximately 500 species of leeches are mostly freshwater inhabitants, although a few have invaded the terrestrial environment, and a few have secondarily invaded marine habitats. The most familiar forms are the ectoparasitic leeches, but many leeches are predators of other worms, insect larvae, snails , and other small invertebrates.
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