a) initially, likely involved the coelom and all organ systems (skin, body wall muscles and nerves, circulatory, excretory and reproductive)

b) 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 and posterior regions of the trochophore larvae

c) new segments are added just anterior to the pygidium

d) each segment has its own coelom; gut is supported by mesentaries

e) two important consequences: i) functionally significant- allows local control of shape in soft-bodied forms, ii) developmentally significant- allows segments to develop independently

f) tagmatization- refers to blocks of segments, structurally distinct from other segments, that perform different functions

a) long, fully segmented worms with complete gut and terminal anus

b) includes most of the largest, worm-like invertebrates (up to 3m!)

c) in addition to a segmented coelom, they have a mostly closed circulatory system (pumping hearts and capillaries)

d) paired, segmental proto- or meta-nephridial systems; nephrostome lies anterior to the segment bearing the nephridial tubule

e) segmented nervous system; primitively paired ventral cords and ganglia

a) most of the anatomical diversity is in the head region and parapodia:

- the prostomium may be complex with tentacles and sense organs; the peristomium surrounds the mouth and lacks parapodia
- many possess paddle-like parapodia on each segment that are bilobed (notopodium and neuropodium), bear many setae (can have many forms)

b) parapodia (including acicula and oblique muscles) and longitudinal muscle bundles permit walking, rapid crawling or swimming

c) reproduction: transient gonads arise from coelom lining; free spawn; some form epitokes= spawning machine; trochophore larvae

d) taxonomic relations unclear; split into 2 "groupings of convenience":

- Errantia: active forms; well developed parapodia and sense organs; many similar segments; eversible pharynx; epitoky common
- Sedentaria: burrow or tube dwelling; reduced parapodia and sense organs; fixed pharynx; strong tagmatization in some; epitoky rare