Bio Review Notes #85
ANNELID WORMS

The Annelida are worms with thorough segmentation of the body.

 Metamerism: Division of the body into numerous similar segments.

Phylum Annelida: Segmented worms. Complete digestive tract (with both mouth and anus) runs nearly the entire length of the body. Outer covering of chitin is thin, flexible, and prevents fluid loss. The true coelom and most internal organs are segmentally arranged. Blood cicrulates in closed vessels only. Advanced excretory organs (nephridia) are present. Some ability to regenerate missing parts after injury.

Locomotion in annelids (controlled separately in each segment):
  • Each segment contains a walled-off portion of the body cavity.
  • Muscles parallel to the body axis can shorten segments; these segments swell and anchor into the surrounding sand or soil.
  • Muscles perpendicular to the body axis will lengthen body segments and cause them to push forward.
  • Nervous system produces rhythmic waves of shortening and waves of lengthening among the segments.
  • Small bristles (setae) may help anchor the shortened segments.
Class Polychaeta: Largest group, mostly marine. Sense organs and nervous system highly developed; several setae per segment.

Class Oligochaeta (earthworms): Poorly developed head; only one pair of setae per segment. Important to soil because their digestive wastes leave behind soil nutrients and their tunnels let air reach plant roots.

Class Hirudinea (leeches): Mostly parasitic, live in fresh water, attach to the outside of animals and suck blood. Leeches have degenerate anatomy: fewer sense organs, fewer segments, etc.


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