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 with polychaetes: pro- and peristomium; highly segmented body and compartmented coelom; paired, segmental ventral nerve cords
d) differ from polychaetes: no parapodia; few setae; small, simple prostomium (no sensory appendages); 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) copulate to transfer sperm; direct development in cocoon (no larva)
a) share 5 traits with oligochaetes: no parapodia; no prostomial appendages; hermaphroditic; permanent gonads in only a few segments; a clitellum (seasonal, so not always visible)
b) differ from oligochaetes in 5 ways: no setae; coelom not compartmented; body dorso-ventrally compressed; anterior and posterior suckers formed by fusion of segments; constant number of body segments (33 or 34 throughout class)
c) move in an "inchworm" fashion using suckers
d) some have eversible proboscis or jaws
e) several adaptations for blood feeding in ectoparasitic forms:
Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)