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CHAPTER 17
Annelids
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Characteristics
Diversity
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Annelids exhibit segmentation or metamerism
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Each unit contains components of most organ
systems
Evolution of metamerism allowed:
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Increased burrowing efficiency
more sophisticated nervous system
Provided a safety factor
 If one segment failed, others could still function
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Characteristics
Classification
 Class Oligochaeta (Eathworms)
 Class Hirudinida (leeches)
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Phylum Annelida
Characteristics
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About 15,000 species
Coelomates - 1st phyla to have true coelom
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Platyhelminthes = acoelomate
Nematoda = pseudocoelomate
Ectoderm
Mesoderm
Endoderm
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Phylum Annelida
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Triploblastic Protostome
Complete Gut
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Platyhelminthes - incomplete
Nematodes- complete
Have a Circulatory System - closed
 Heart-like structure : aortic arches
 Other worms - no circulatory system
Setae (se’-tah)
 Tiny bristles called setae
 Absent in leeches
 Short setae anchor segments in earthworms
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Prevent it from slipping backward
Long setae help aquatic worms swim
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Phylum Annelida
Body Plan
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Body Wall
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Head is composed of
 Prostomium
Terminal portion bearing the anus is the pygidium
Head and pygidium are not considered metameres
(segments)
New metameres form in front of the pygidium
Surface is covered with an epidermis and a thin
outer layer cuticle
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Marine Annelids
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Coelomates
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Phylum Annelida
Phylogeny
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Traditionally, annelids are divided among 3
classes
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Class Polychaeta (marine)
Class Oligochaeta (earthworms)
Class Hirudinida (leeches)
Oligochaeta and Hirudinida
 Characterized by reproductive structure
called a clitellum
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Clade clitellata
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Clade Clitellata
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Class Oligochaeta and Class Hirudinida
Form reproductive structure called a
clitellum
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Permanent in oligochaetes but visible only
during reproductive season in leeches
Hermaphroditic (monoecious) animals
Young develop inside a cocoon
secreted by the clitellum, and emerge
as small worms
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Clade Clitellata
Class Oligochaeta- earthworm
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Diversity
Over 3000 species
 Nearly all have setae (bristle like hairs)
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Form and Function
Sometimes called “night crawlers”
 Burrow in moist rich soil and usually live in
branched interconnected tunnels
 Damp, rainy weather: Remain near surface
 Dry weather: Burrow deep underground and
go dormant coiled in a slime chamber
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Clade Clitellata
Nutrition
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Scavengers, feeding on decayed organic matter,
leaves, etc.
Food moistened by mouth and drawn in by a
sucking action of muscular pharynx
Food is stored in a thin-walled crop
Muscular gizzard grinds food into small pieces
Digestion and absorption occur in intestine
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Clade Clitellata
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Circulation and Respiration
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Coelomic fluid and blood transport food, wastes, and
respiratory gases
Blood circulates in a closed system
Dorsal vessel and Ventral vessel present.
Heart consists of 5 pairs of aortic arches
No special gaseous exchange organs
 Gas exchanged across body surface
Excretion
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Each segment, except the 1st three, have a pair of
metanephridia (excretory organ) - releases waste into
nephridiopore
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Excretory System
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Nervous System: Brain, Nerve Cord, Peripheral
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Nerve Cord
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Clade Clitellata
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Reproduction and Development
Monoecious (hermaphroditic)
 Reproductive structures are located in
segments 9 through 15
 Sperm produced by testes mature in
seminal vesicles and pass into sperm
ducts
 Eggs are discharged by ovaries
 Two pairs of seminal receptacles receive
and store sperm
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Clade Clitellata
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Mate at night during warm, moist weather
Aligning in opposite directions, ventral surfaces
together
Mucus secreted by clitellum holds worms together
Sperm from each worm are transported to the
seminal receptacles of the other along seminal
grooves
After mutual copulation, each worm secretes a
mucus tube to form a cocoon
Cocoon passes forward and eggs, and sperm are
added
Fertilization and embryogenesis occur in cocoon
Young worms emerge from cocoon
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Clade Clitellata
Class Hirudinida: Leeches
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Hirudinea
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34 segments, lack setae but possess anterior and posterior
suckers
Diversity
 Most freshwater, few marine or live in moist terrestrial environments
 More common in the tropics temperate zones
 Vary in color: black, brown, red, and olive green
 Some carnivores feeding on small invertebrates
 Others are temporary or permanent parasites
 Hermaphroditic
 Form a clitellum during breeding season
 Secretes a cocoon for reception of eggs
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Largest
Leech
Gut specialized
for storage of
large quantities
of blood
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2 suckers:
Posterior
and Anterior
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Clade Clitellata
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Nutrition
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Not all are parasites, many are predators
Freshwater leeches have a proboscis (pharynx) for
ingesting small invertebrates as well as to suck
blood
Some terrestrial leeches feed on insect larvae,
earthworms, and slugs
Other terrestrial leeches climb trees or bushes to
reach warm-blooded vertebrates such as baby
birds
Most are fluid feeders that prefer tissue fluids and
blood pumped from open wounds
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Medicinal Leech:
Used to relieve congestion in the veins due to injury.
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