• 1) Segmentation= Metamerism is a trait of remaining protostomes
  
  a) often involves the coelom & all organ systems (skin, body wall muscles & nerves, circulatory, excretory & 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 & 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

• 2) ANNELIDA includes approx. 11,500 marine, freshwater & terrestrial spp.
  
  a) long, fully segmented worms with complete gut & terminal anus
  b) includes most of the largest, worm-like invertebrates
  c) in addition to a segmented coelom, they have a mostly closed circulatory system (capillaries & pumping hearts)
  d) paired, segmental proto- or meta-nephridial systems; nephrostome lies anterior to the segment bearing the nephridial tubule

• 3) Cl. POLYCHAETA (marine segmented worms, approx. 8,000 spp)
  
  a) most of the anatomical diversity is in the head region & parapodia:
  
  - the prostomium may be complex with tentacles & sense organs; the peristomium surrounds the mouth and lacks parapodia
  - many possess paddle-like parapodia on each segment that are bilobed (notopodium & neuropodium) and possess many setae
  
  b) parapodia (including acicula & oblique muscles) and longitudinal muscle bundles permit walking, rapid crawling or swimming
  c) reproduction: gonads transient, from lining of coelom; free spawn
  d) taxonomic relations unclear; split into 2 "groupings of convenience":
  
  Errantia- active forms; well developed parapodia & sense organs; many similar segments; eversible pharynx; epitoky common
  Sedentaria- burrow or tube dwelling; reduced parapodia & sense organs; fixed pharynx; strong tagmatization in some; epitoky rare

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