Annelids

{ Characteristics }

Annelids have segmented bodies and a true coelom that is line with tissue derived from mesoderm.

Reproduction

Annelids have one mean of reproducing. They reproduce sexually.

Most annelids reproduce sexually, even though they are hermaphroditic. They exchange sperm, and when the eggs are ready for fertilization, a clitellum secretes a mucus ring where the eggs and sperm are released. Fertilization take place in the ring. The ring slips off the worm’s body and forms a protective cocoon where worms hatch from a couple weeks later.

Examples of Annelids

Annelids are divided into 3 classes: Oligochaeta, Hirudinea, and Polychaeta. The class Oligochaeta contain earthworms and their relatives. They are annelids that typically have streamlined bodies and relatively few setae compared to polychaetes. Most live in soil or fresh water. The class Hirudinea contains leeches. Leaches are typically external parasites that suck blood and body fluids of their host. In the Middle Ages, leeches were considered to leech the excess blood from a host to remove a disease. The class Polychaeta are marine annelids that have paired paddlelike appendages tips with setae.

Vocabulary

septum - internal body wall between the segments of an annelid’s body

seta - bristle attached to the segments of many annelids

crop - part of the digestive system in which food can be stored

gizzard - part of the digestive system in which food is ground into smaller pieces

closed circulatory system - system in which blood is contained within a network of blood vessels

gill - filamentous organ in aquatic animals specialized for the exchange of gases with water

nerphridium -excretory system of an annelid that filters fluid in the coelom

clitellum - band of thickened, specialized segments in annelids that secretes a mucus ring into which eggs and sperm are released

Works Cited

http://www.freewebs.com/invertebratezoology/Earthworm1.jpg

http://www.ukdivers.net/images/bristle.jpg

http://www.earthlife.net/inverts/images/annelids/hirudinea.jpg

http://www.science.gu.se/digitalAssets/1307/1307803_paranaitis_katoiwebb.jpg