CHAPTER 17
Annelids
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Characteristics
Diversity
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Annelids exhibit segmentation or metamerism
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Bodies composed of serially repeated units
Each unit contains components of most organ
systems
Evolution of metamerism allowed much greater
complexity in structure and function
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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
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Segmented worms living in marine,
freshwater, and moist terrestrial
habitats
Include marine bristle worms, leeches,
and earthworms
Classification
Class Polychaeta (scale, fire, tube worms)
 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 tru coelom
Segmentation
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Body segments marked by circular grooves
called annuli
Metamerism
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Repetition of organs in segments called metameres or
somites
Septa - internal tissue separate segments
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Phylum Annelida
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Setae
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Tiny bristles called setae
Absent in leeches
Short setae anchor segments in earthworms
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Primarily marine
Class Oligochaetes (earthworms) and
Hirudinida (leeches)
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Long setae help aquatic worms swim
Class Polychaetes
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Prevent it from slipping backward
Freshwaters, or terrestrial soils
Many leeches are predators
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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
Strong circular and longitudinal muscles underlie
the body wall
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Coelomates
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Phylum Annelida
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Hydrostatic Skeleton
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Except in leeches, coelom is filled with fluid and
serves as a hydrostatic skeleton
Fluid volume remains constant
Contraction of longitudinal and circular muscles
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Causes body to shorten and expand, narrow and
lengthen
By separating this force into sections, widening
and elongation move the whole animal
Alternate waves of contraction, allow efficient
burrowing
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Phylum Annelida
Phylogeny
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Traditionally, annelids are divided among 3
classes
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Class Polychaeta
Class Oligochaeta (earthworms)
Class Hirudinida (leeches)
Oligochaeta and Hirudinida
 Characterized by reproductive structure
called a clitellum
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Clade clitellata - members are class Oligochaeta
and class Hirudinida
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Clade Clitellata
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Class Oligochaeta and Class Hirudinida
Form reproductive structure called a
clitellum
Ring of secretory cells found in a band
around the body
 Permanent in oligochaetes but visible only
during reproductive season in leeches
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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
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Over 3000 species
Occur in habitats from soil to freshwater
Few are marine or parasitic
Nearly all have setae (bristle like hairs)
Form and Function
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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
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Setae
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Bristlelike rods moved by tiny muscles
Project outward through small pores in cuticle
Aid anchoring by digging into walls of burrow
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
Typhlosole increases surface area
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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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Clade Clitellata
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Nervous System and Sense Organs
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Central nervous system and peripheral nerves
Pair of cerebral ganglia connect around the
pharynx to the ganglia of the ventral nerve cord
Fused ganglia in each segment contain both
sensory and motor fibers
Lack eyes but have many photoreceptors in the
epidermis
Free nerve endings in tegument are tactile
structures
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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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General Behavior
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Avoid bright light
Chemical stimuli are important in locating food
Limited learning ability
 Primarily trial-and-error learning
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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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Freshwater
Oligochaetes
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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
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Clade Clitellata
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Form and Function
Usually have a fixed number of segments
 Lack distinct compartments
 No septa
 No setae
 Developed suckers for attachment
 Gut specialized for storage of large
quantities of blood
 Most use suckers to attach and inch along
surfaces
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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
Some parasitic leeches leave a host during
breeding season
 Fish leeches may remain on a host permanently
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Medicinal Leech:
Used to relieve congestion in the veins due to injury.
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