BIO2231 Chapter Notes -Ventral Nerve Cord, Nephridium, Aortic Arches
Document Summary
Annelida are worms whose bodies are divided into similar segments (also called metamers) arranged in linear series and externally marked by circular rings called annuli. Body segmentation (metamerism) is a division of this body into a series of segments, each of which contains similar components of all major organ systems. In annelids, the segments are delimited internally by septa. Annelids are sometimes called bristle worms" because, with the exception of leeches, most annelids bear tiny chitinous bristles called setae. Short, needlelike setae help anchor segments during locomotion and long, hairlike setae aid aquatic forms in swimming. Since many annelids burrow or live in secreted tubes, stiff setae also aid in preventing the worm from being pulled out or washed out of its home. The annelid body typically has a two-part head, composed of a postomium and a peristomium, followed by a segmented body and a terminal portion called the pygidium bearing an anus.
