ANNELIDA (I): Intro., Polychaeta (Annelida study images;
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a) initially, likely involved 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
a) long, fully segmented worms with complete gut & 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 & 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 & 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: gonads transient, from coelom lining; free spawn; 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
Copyright (c) 2005 by A. Richard Palmer. All rights reserved.
(revised Mar. 3, 2005)