The Phylum Annelida is broken down into three classes: Polychaeta, Oligochaeta, and Hirudinea. The one class that will be focused on is Polychaeta since the other two consist of land dwelling animals. Annelids are worms, most of which have bodies that divided internally and externally into segments that allow them more mobility by enhancing leverage. Most worms gain support for their body from fluid (mostly water) that is contained in a body compartment. This is known as a hydrostatic skeleton.