1) Segmentation= Metamerism; a trait of all remaining protostomes
a) primitively involves the coelom & all organ systems (skin, body wall muscles & nerves, circulatory, excretory & reproductive)
b) the mouth lies behind an apical acron or prostomium; the anus lies in a terminal pygidium; neither are considered true segments but derive from the anterior & posterior regions of the larvae
c) new segments are added just anterior to the pygidium
d) two important consequences: I) functionally significant- allows local control of shape in soft-bodied forms, II) developmentally significant- allows segments to develop independently
e) tagmatization- blocks of segments structurally distinct from other segments that perform different functions
2) Annelida includes 3 classes: Polychaeta, Oligochaeta, Hirudinea
a) long, segmented worms with sub-terminal mouth & terminal anus
b) attain the largest size of any worm-like invertebrates
c) have a closed circulatory system (capillaries & pumping hearts)
d) have paired, segmental metanephridial systems; the nephrostome is anterior to the segment bearing the nephridial tubule