Download LECTURES FOR ZOO 1010—CHAPTER 1

LECTURES FOR ZOO 1010—CHAPTER 11
Segmented Worms: Annelids
BASED ON HICKMAN ET AL. ANIMAL DIVERSITY
3rd Edition
Chapter Prologue—Dividing Up the Body: Precise control of movement became
possible with the evolution of a body space segmented into partitions. When septa
partitioned the coelom into a series of compartments, various organs systems, such as
the circulatory, nervous, and excretory systems, were repeated in each segment. This
type of body plan is termed metamerism. Complexity of structure and function
became possible with the appearance of metamerism. Fine control of movement was
accompanied by evolution of more sophisticated nervous systems. Metamerism also
allows for redundancy in some systems, providing a safety factor; if one segment
should fail or be lost, others could still function. Metamerism is also seen in
arthropods, where it may be an independent development, and among chordates,
where it indeed is an independent development.
The phylum Annelida contains the segmented worms, of which there are about
15,000 species. The most familiar of these animals are earthworms and leeches. The
largest group, however, consists of the marine annelids of the class Polychaeta,
comprising about two-thirds of the phylum. Annelids are eucoelomate protostomes,
with, therefore, spiral and mosaic cleavage.
Most annelids, save for leeches, possess locomotory-associated structures called
setae, used in burrowing or swimming.
Ecological relationships—Segmented worms are cosmopoliton in distribution,
occurring in the sea, fresh water, and terrestrial soil. Some polychaetes are burrowers
or tube-builders. Some feed on organic matter in mud through which they burrow;
others are filter feeders. Many polychaetes are predaceous. Freshwater annelids
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burrow in mud or sand, live among vegetation, or are free-swimming. Terrestrial
earthworms burrow through the soil. Some leeches are fluid feeders on blood; others
are carnivorous. Most of them live in fresh water.
Economic importance—Annelids are of indirect economic importance, as a
consequence of their ecological roles. Some are members of grazing or detritus food
chains and serve as prey for other organisms of more direct interest to humans
(earthworms as fish bait, for example). Earthworms assist in the drainage, aeration,
and fertilization of soil. Some marine annelids perform this role in the oceans.
Leeches may have medicinal uses, based on their blood-sucking abilities.
Body Plan:
The typical annelid body has a head or prostomium, a metamerically arranged body,
and a pygidium containing the anus. New segments form directly in front of the
pygidium. The body wall possesses both circular and longitudinal muscles used
for swimming, crawling, or burrowing and is covered by an epidermis and a thin
cuticle. Annelids are schizocoelous. The coelomic compartments are surrounded
by peritoneum, which also forms double-walled mesenteries. Where peritonea of
adjacent segments meet, septa are formed. These septa are pierced by the
digestive, circulatory vessels, and nerve cord. The coelom is filled with a fluid
and serves as a hydrostatic skeleton. Alternating waves of circular and
longitudinal muscular contraction is termed peristaltic contraction and allows for
crawling and burrowing behavior.
Class Polychaeta:
The marine annelids comprise about 10,000 species. Majority range in size from 5 to
10 cm; some, however, are less than a mm in length, whereas others may range to
as much as 3 m. These worms live under rocks, in coral crevices, or in abandoned
shells, or they burrow in mud or sand; some are tube-builders; others are pelagic.
They are important components of marine food chains.
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Polychaetes differ from other annelids in having a well developed head with
specialized sensory structures, parapodia, and no clitellum. The many setae are
arranged in bundles on the parapodia. Parapodia can be specialized for swimming,
respiration, crawling, maintaining position in a burrow, pumping water through a
burrow, and accessory feeding, an illustration of the benefits of metamerism, i.e.,
specialization of segmentally-arranged structures along the length of the body.
Polychaetes are dioecious, with a primitive reproductive system, no clitellum,
external fertilization, and a trochophore larva.
Class Oligochaeta:
About 3000 species of oligochaetes are known; they range tremendously in size and
in the type of habitat they occupy. They have relatively few setae compared to
polychaetes, no parapodia, but do possess a clitellum. They have no head.
Oligochaetes are monoecious (with cross fertilization); development is direct. The
clitellum functions in reproduction, including secretion of mucus to surround two
worms during copulation and secretion of a cocoon to receive the gametes which
undergo fertilization followed by embryonation; a small, juvenile worm emerges
from the cocoon.
Class Hirudinea:
Leeches have a fixed number of body segments (usually 34), anterior and posterior
suckers, and a clitellum. No parapodia are present and the coelom is closely
packed with connective tissue and muscle. Leeches are hermaphroditic and
development is direct. They occur on land, in fresh water, and in marine settings.
Leeches feed mostly on fluids, with many being predators; some are temporary
parasites and others are permanent parasites.
Phylogeny and Adaptive Radiation:
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Phylogeny— Embryological evidence places annelids with mollusks and arthropods
in the Protostomia. Recent molecular evidence suggests that annelids and
mollusks are more closely related to one another (in the superphylum
Lophotrochozoa) than either phylum is to the arthropods (in the superphylum
Ecdysozoa).
Adaptive radiation—The cladistic relationships of the annelid classes appears to be as
follows: Polychaeta is the basal group and is the sister group of all other annelids
(the Clitellata). Oligochaeta is the sister group of the Hirudinida
(Acanthobdellida, Branchiobdellida, and Hirudinea).