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Annelida Classes
  • Polycheata (marine)
  • Oligochaeta (earthworms)
  • Hirudinea (leeches)

  • Annelida is a group commonly referred to as segmented worms, and they are found worldwide from the deepest waters/marine sediments to the soils in our city parks and yards.
  • Through most of the 20th century Annelida was split into three major groups; Polychaeta, Oligochaeta (also known as earthworms) and Hirudinea (also known as leeches.)
  • Earthworms and leeches are the familiar annelids for most people, but polychaetes (annalid worms) make up the bulk of the different types of Annelida and are found in nearly every marine habitat.
  • There are even pelagic (living in or near water) polychaetes that swim or drift, preying on other plankton, and a few groups occurring in fresh water and moist terrestrial surroundings.


Annelids are all bilaterally symmetrical animals. They range in size from much less than 1 mm in length to more than 3 m. Nearly all annelids have a fluid-filled cavity between the outer body wall and the gut, and this is referred to as a coelom. The coelom is often used as a storage area for gametes and acts as a hydrostatic skeleton for locomotion.

Important Key Facts About Annelida

  • There are about 12,400 species of annelid, all of which are vermiform, or soft bodied.
  • The repition of segments in annelid is called metamerism
  • Annelid segments are seperated by septa. Septa is defined as "a thin partition or membrane that divides two cavities or soft masses of tissue in an organism."
  • Most polycheate annelids have seperate sexes. (male and female)