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Marine Phyla Pages -- Coastal Carolina University ANNELIDA |
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Annelids ____________________
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Defining
Characteristics
1
Segmented
worms make up the Phylum Annelida. The phylum includes earthworms and their
relatives, leeches, and a large number of mostly marine worms known as
polychaetes. Annelids are coelomate
animals; they have a fluid-filled body cavity in which the gut and other
organs are suspended. Systematics
2
The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Gases are exchanged through the skin, or sometimes through specialized gills or modified parapodia. Each segment typically contains a pair of nephridia. The nervous system includes a pair of cephalic ganglia attached to double nerve cords that run the length of the animal along the ventral body wall, with ganglia and branches in each segment. Annelids have some combination of tactile organs, chemoreceptors, balance receptors, and photoreceptors; some forms have fairly well developed eyes, including lenses. Annelids with a clitellum (a swelling towards the head of the animal, where the gonads are located) are classified in the Clitellata, which is further divided into Hirudinea (leeches) and Oligochaeta (including the earthworms). The Polychaeta lack a clitellum and have parapodia, paddle-like appendages with numerous bristles or chaetae. One aberrant group, the Myzostomaria, parasitizes echinoderms and is sometimes considered a separate class of annelids
Feeding 1
Annelids
have radiated into a number of niches. Some are parasitic, notably the
leeches and myzostomarians; others filter-feed or prey on other
invertebrates. Habitat
3
Annelids
are found worldwide from the deepest marine sediments to the soils in our
city parks and yards. Other
Information
1
The most significant ecological role played by annelids is reworking of soil and sediments. Many polychaetes and oligochaetes, and even a few leeches, are burrowers that constantly rework the sediment through which they burrow; in addition, they may ingest and excrete large quantities of sediments or soils. Robison (1987) notes that some sandy beaches may harbor 32,000 burrowing annelids per square meter, which collectively may ingest and excrete 3 metric tons of sand per year. Annelida Links
1. http://www.ucmp.berkeley.edu/annelida/annelida.html 2. http://animaldiversity.ummz.umich.edu/annelida.html 3. http://tolweb.org/tree?group=Annelida&contgroup=Bilateria |