Phylum ANNELIDA: Intro., Polychaeta
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
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 proto- or meta-nephridial systems; the nephrostome lies anterior to the segment bearing the nephridial tubule
e) phylogenetic relations are almost wholly unresolved
a) the coelom is subdivided laterally by longitudinal mesentaries
b) the prostomium may be complex with tentacles & sense organs; the peristomium surrounds the mouth and lacks parapodia
c) many possess paddle-like parapodia on each segment that are bilobed (notopodium & neuropodium) and possess many setae
d) parapodia (including acicula & oblique muscles) and longitudinal muscle bundles permit walking, rapid crawling or swimming
e) often split into 2 groups of convenience: 'errantia' & 'sedentaria'
f) errantia- active forms with well developed parapodia & sense organs and an eversible pharynx
g) sedentaria- burrow or tube dwelling forms with reduced parapodia & sense organs and a fixed pharynx
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Copyright © 1997 by A. Richard Palmer. All rights reserved.(revised March 16, 1998)