Annelids are worms with segmented bodies, they have a true
coelom that is completely lined with mesoderm.
Oligochaetes are annelids that typically have streamlines
bodies and relatively few setae compared to polychaetes. Most oligochaetes live
in soil or fresh water. Leeches are typically external parasites that suck the
blood and body fluids of their hosts.
Polychaetes are marine annelids that have paired, paddle
like appendages tipped with setae.
Reproduction in annelids
Most annelids reproduce sexually. Some species use external fertilization and have separate sexes ( either male or female). Other annelids, such as earthworms and leeches are hermaphrodites, each worm produces both sperm and eggs. Individuals rarely fertilize their own eggs, instead two worms attach to each other and exchange sperm, then store it in special sacs. When eggs are ready for fertilization, a clitellum or band of thickened, specialized segments, secretes a mucus ring into which eggs and sperm are released. fertilization takes place within this ring, which then splins off the worms body and forms a protective cocoon. Young worms hatch weeks later.
Respiration in annelids
Aquatic annelids often breathe through gills. A gill is a filamentous organ specialized for the exchange of gases underwater. Land-dwelling annelids take in oxygen and give off carbon dioxide through their moist skin. These annelids secrete a thin protective cooating of mucus, which keeps their skin moist.
