ANNELIDA (II): Oligochaeta, Hirudinea
(Annelida study images;
a) remarkably little morphological diversity; mainly detritivores
b) Darwin estimated they move 18 tons of soil/acre/year to surface!
c) share features with polychaetes: pro- and peristomium; highly segmented body and compartmentalized coelom; paired, segmental ventral nerve cords
d) differ no parapodia; few setae; small, simple prostomium (no sensory structures); longitudinal muscles not in bundles; hermaphroditic; permanent gonads in few segments; clitellum secretes a cocoon to hold eggs
e) burrow via retrograde peristaltic waves of body-wall muscles
f) all have direct development in a cocoon (no larval stage)
a) share 5 traits with oligochaetes: no parapodia; no prostomial appendages; hermaphroditic; permanent gonads occur in only a few segments; a clitellum (seasonal so not always visible)
b) differ from oligochaetes in 5 ways: no setae; coelom not compartmentalized; body dorso-ventrally compressed; anterior and posterior suckers formed by fusion of segments; constant number of body segments (33 or 34 throughout class)
c) some have eversible proboscis or jaws
d) several adaptations for blood feeding in ectoparasitic forms:
Copyright (c) 2013 by A. Richard Palmer. All rights reserved.
(revised Jan. 3, 2013)