ANNELIDA (I): Introduction, Polychaeta (Annelida study images;
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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 prostomium (= acron); 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) metamerism has 2 big consequences: i) functionally significant- allows local control of shape in soft-bodied forms, ii) developmentally significant- allows segments to develop independently
f) tagmatization- the body has blocks of segments (tagmata), distinct from other blocks of 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:
- prostomium may be complex with tentacles and sense organs; peristomium surrounds mouth, may have tentacles, no parapodia
- have paired, 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: sexes separate; transient gonads from coelom wall; free spawn; some form epitokes= spawning machine; trochophore larvae
d) taxonomic relations of higher Polychaeta fairly clear:
- Errantia: active; often predatory; well developed parapodia and sense organs; similar segments; eversible pharynx; epitoky common
- Sedentaria: burrow or tube dwelling; particle feeders; reduced parapodia and sense organs; fixed pharynx; strong tagmatization in some; epitoky rare
Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016
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