What are Annelids?
Annelids, members of phylum Annelida, are a group of worms with about 18,000 species, including the familiar earthworm. Annelids have a fossil record stretching back to the Cambrian, and are distinguished from other organisms called “worms” by the presence of a body cavity (coelom) and true segmentation, both of which give them an evolutionary advantage. Annelids are the most complex organisms capable of full regeneration if they are cut in half, and indeed are able to reproduce asexually by releasing an end of their tail, which subsequently grows into a complete organism. Annelids can also reproduce sexually. There are four classes of annelids: polychaete worms, marine annelids which make up the majority of all species (more than 10,000), clitellates, a large group which includes leeches and earthworms, haplodrils, simple marine worms, and myzostomids, small parasites of sea lilies. Polychaetes are characterized by lateral outgrowths called parapodia, which are covered in chitinous br
They are segmented worms that make up the phylum Annelida. These worms include earthworms, relatives to earthworms, leeches, and a wide variety of marine worms known as polychaetes Special Features of the Phylum Annelids offer unique insights on the early evolution of spiral cleavage, anteroposterior axis formation, body axis segmentation, and head versus trunk distinction. Annelids undergo a mode of embryonic development known as spiral cleavage. The basic Annelid body plan is one of a head followed by a long thin body of numerous similar segments ending in a small tail. They are also abundant in all habitats. Annelids are one of the great branches of animal life with many thousands of species.
What is an earthworm? Phylum Annelida contains the segmented wormsClasses Oligochaeta (earthworms and freshwater annelids), Polychaeta (bristled marine sandworms), and Hirudinea (leeches)almost all of which are free-living (rather than internal parasites, as is the case with many other worms). There are over 8,700 known annelid species. Polychaetes have more hairs or setae than the familiar oligochaetes, and polychaetes also often have fleshy appendages called parapodia on their body segments. Unlike oligochaetes, polychaetes often have well-developed eyes, chemoreceptive antennae, and other anterior sensory organs. Polychaetes generally burrow in the sand or build tubes to live in; they may trap tiny animals or detritus with ciliated oral tentacles, or else filter food from water they pump through their burrow. Leeches lack the setae found in other annelids. Almost all leeches live in fresh water. Some leeches are parasitic (sucking blood or other body fluids from other animals), and
