Download Clade Clitellata

CHAPTER 17
Annelids and
Allied Taxa
17-1
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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 by permitting
independent movement of segments
Evolution of a 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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Cleavage is spiral mosaic
Mesoderm forms from derivatives of the 4d
cell
Coelom forms by schizocoely
Share a trochophore as the ancestral larval
form
Segmented worms living in marine,
freshwater, and moist terrestrial habitats
Include marine bristle worms, leeches, and
earthworms, pogonophorans, and
vestimentiferan worms
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Characteristics
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Members of phylum Echiura and Sipuncula
are benthic marine animals with
unsegmented bodies
Molecular sequence data place echiurans
within phylum Annelida
Echiurans
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Sipunculans
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Sister taxon to Annelida
Sister taxon to a clade composed of Annelida and
Echiura
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17-5
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Phylum Annelida
Characteristics
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About 15,000 species
2/3 are the more obscure marine worms.
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 separate segments
Found in arthropods, probably homologous with
annelids, and in vertebrates, where it evolved
independently
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Phylum Annelida
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Setae
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Tiny chitinous bristles called setae
Absent in leeches
Short setae anchor segments in earthworms
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Primarily marine and usually benthic
Oligochaetes and leeches
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Long setae help aquatic worms swim
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
 Perstonium
Terminal portion bearing the anus is the pygidium
Head and pygidium are not considered metameres
New metameres form in front of the pygidium
Surface is covered with an epidermis and a thin
outer layer of non-chitinous cuticle
Strong circular and longitudinal muscles underlie
the body wall
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17-9
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Phylum Annelida
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Coelom
 Forms by schizocoely
 Peritoneum (mesodermal epithelium) lines
body wall and forms dorsal and ventral
mesenteries
 Peritonea of adjacent segments meet to
form the septa
 Gut and longitudinal blood vessels extend
through the septa
17-10
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17-11
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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 muscles
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Contraction of circular muscles
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Causes body to shorten and expand
Causes body to narrow and lengthen
By separating this force into sections, widening
and elongation move the whole animal
Alternate waves of contraction, or peristalsis,
allow efficient burrowing
Swimming annelids use undulatory movements
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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
Class Hirudinida
Polychaeta is a paraphyletic class
Oligochaeta and Hirudinida form a
monophyletic group called Clitellata
 Characterized by reproductive structure
called a clitellum
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Phylum Annelida
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Some now consider Clitellata to be an
annelid class containing oligochaetes and
leeches as orders
We retain the three classes and consider
Clitellata a clade whose members are class
Oligochaeta and class Hirudinida
Class Oligochaeta is a paraphyletic group
because ancestors of leeches arose from
within it
17-14
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Class Polychaeta
Diversity
Largest class of annelids
 More than 10,000 species, mostly marine
 Vary from 1 mm to 3 meters long
 Differentiation of some somites
 More specialization of sensory organs than in
clitellates
 Tolerate a wide range of salinity
 Warmer regions have more freshwater
polychaetes
 Some live in crevices, others inhabit tubes, or
are pelagic
17-15
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Class Polychaeta
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Important part of marine food chains
Well-differentiated head with sense organs
Paired appendages called parapodia on most
segments
No clitellum
Many setae arranged in bundles on parapodia
Sedentary polychaetes mainly tube-living
Errant polychaetes may be free-moving,
burrowing, or crawling
Clamworm Nereis is an example of a
predatory polychaete with jaws on an
eversible, muscular pharynx
17-16
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17-17
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Class Polychaeta
Form and Function
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Prostomium may or may not be retractile
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Often bears eyes, tentacles, and sensory palps
Surrounds mouth and may have setae, palps, or
chitinous jaws
Ciliary feeders may bear a tentacular crown
that opens like a fan but can be withdrawn
into the tube
Most segments of trunk bear parapodia with
lobes, cirri, setae, and other parts
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Aid crawling, swimming, and anchor worm in a
tube
Usually the chief respiratory organ although the
worm may also possess gills
 Amphritite and Arenicola
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17-19
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Class Polychaeta
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Nutrition
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Polychaetes have a foregut, midgut and hindgut
Foregut has a stomodeum, pharynx, and anterior
esophagus lined with cuticle
Midgut derived from mesoderm secretes enzymes
and absorbs nutrients
Short hindgut derived from ectoderm and leads to
anus on the pygidium
Errant polychaetes are predators or scavengers
Sedentary polychaetes feed on suspended
particles or particles in sediment
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17-21
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Class Polychaeta
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Circulation and Respiration
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Most have parapodia and gills for gaseous
exchange
 Others use the body surface
Circulation varies
 In Nereis a dorsal vessel carries blood forward
and a ventral vessel carries blood posteriorly
 Blood flows across between these major vessels
in networks around the parapodia and intestine
In some, septa are incomplete and coelomic fluid
serves circulatory function
Many polychaetes have respiratory pigments
 Hemoglobin, chlorocruorin or hemerythrin
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Class Polychaeta
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Excretion
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Excretory organs vary, from protonephridia to
metanephridia, and mixed forms
One pair per metamere
Inner end (nephrostome) opens into the coelomic
cavity
Coelomic fluid enters the nephrostome
Selective resorption occurs along the nephridial
duct
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Class Polychaeta
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Nervous System and Sense Organs
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Dorsal cerebral ganglia connect to subpharyngeal
ganglia by a circumpharyngeal commissure
Double ventral nerve cord runs length of the worm
with ganglia in each metamere
Sense organs include
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Eyes, nuchal organs and statocysts
Eyes vary from simple eyespots to well-developed imageresolving eyes similar to mollusc eyes
Alciopid eyes have accessory retinas specialized for the
wavelengths that penetrate deep seas
Nuchal organs are ciliated sensory pits that are probably
chemoreceptive
Some burrowing and tube-building polychaetes use
statocysts to orient their body
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Class Polychaeta
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Reproduction and Development
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Polychaetes have no permanent sex organs
Monoecious
Gonads appear as simple temporary swellings of
peritoneum
Gametes are shed into coelom and exit by
gonoducts, metanephridia, or rupturing of the
body
Fertilization is external and the early larva is a
trochophore
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17-26
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17-27
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Class Polychaeta
Representative polychaetes
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Clam Worms: Nereis
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Errant polychaetes
Live in mucus-lined burrows near low tide level
Come out of hiding places at night to search for
food
Prostomium bears a pair of palps sensitive to
touch and taste, a pair of short sensory tentacles,
and two small dorsal eyes sensitive to light
Peristomium has a ventral mouth, a pair of jaws,
and four pairs of sensory tentacles
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Class Polychaeta
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Parapodia
 Each has two lobes
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One or more chitinous spines (acicula) support
each lobe
 Abundant blood vessels assist respiration
 Function in creeping and swimming
 Oblique muscles in each somite
 Undulatory movements of the body provide freeswimming and burrow-pumping actions
Feed on small animals, other worms, and larval forms
Food moved through alimentary canal by peristalsis
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Dorsal notopodium
Ventral neuropodium
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Class Polychaeta
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Scale worms
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Belong to the family Polynoidae
Flattened bodies are covered with broad scales
Some are large, all are carnivores and some are
commensals in burrows of other organisms
Fireworms
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Have hollow, brittle setae that contain poisonous
secretions
Feed on cnidarians
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17-31
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Class Polychaeta
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Tubeworms
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Fanworms
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Tube-dwellers
May line their burrows with mucus
Use cilia or mucus to obtain food
Unfurl tentacular crowns to feed
Food moved from radioles to mouth by ciliary action
Parchment Worms
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Lives in a U-shaped tube
Modified segments pump water through tube
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17-33
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17-34
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17-35
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Clade Siboglinidae (Pogonophorans)
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Formerly members of phylum Pogonophora
(beardworms)
Discovered in 1900
Characteristics
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150 species described
Most are small, less than 1 mm in diameter
Giant beardworms that live in deepwater
hydrothermal vents are 3 m long and 5 cm in
diameter
17-36
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Clade Siboglinidae (Pogonophorans)
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Most live in mud on ocean floor at depths of
100 to 10,000 m
Sessile animals that secrete and live in long
chitinous tubes
Tubes have general upright orientation in
bottom sediments
Tubes are generally three or four times the
length of the animal
17-37
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17-38
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Clade Siboglinidae (Pogonophorans)
Body
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Long cylindrical body covered with cuticle
Divided into a short anterior forepart, a long
slender trunk, and a small, segmented
opisthosoma
Tentacles located on a cephalic lobe
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Hollow extensions of the coelom and bear minute
pinnules
Lie parallel to one another, enclosing a cylindrical
intertentacular space
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Clade Siboglinidae (Pogonophorans)
Internal body
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No mouth or digestive tract
Mode of digestion puzzling
Nutrients such as glucose and amino acids
absorbed from seawater through pinnules
and microvilli of tentacles
Most energy derived from a mutualistic
relationship with chemoautrophic bacteria
that oxidizes hydrogen sulfide
Trophosome, derived embryonically from
midgut, houses the bacteria
17-40
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17-41
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Clade Siboglinidae (Pogonophorans)
Reproduction and Development
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Sexes are separate
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Research suggests that cleavage is unequal
and atypical
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Pair of gonads and gonoducts in trunk section
Appears to be spiral
Coelom formed by schizocoely
Embryo
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Worm-shaped and ciliated
Poor swimmer
Probably carried by water currents until it settles
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17-43
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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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Ring of secretory cells found in a band around the
body
Permanent in oligochaetes but visible only during
reproductive season in leeches
Members are derived annelids that lack
parapodia
Hermaphroditic (monoecious) animals that
exhibit direct development
Young develop inside a cocoon secreted by
the clitellum, and emerge as small worms
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Clade Clitellata
Class Oligochaeta
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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 bear setae
 Fewer in number than in polychaetes
Form and Function
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Sometimes called “night crawlers”
Burrow in moist rich soil and usually live in
branched interconnected tunnels
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17-46
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Clade Clitellata
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Damp, rainy weather: Remain near surface
Dry weather: Burrow deep underground and go
dormant coiled in a slime chamber
Peristaltic movements
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Contractions of circular muscles in the anterior end
lengthen the body, pushing the anterior end
forward where it anchors
Anchoring is accomplished by contraction of the
longitudinal muscles in forward segments
Causes the segments to become short and wide,
pushing against the burrow
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Clade Clitellata
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Setae
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Bristlelike rods set in a sac and 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, refuse, etc.
Food moistened by mouth and drawn in by a
sucking action of muscular pharynx
Calcium in soil leads to high blood Ca+2
 Calciferous glands along the esophagus reduce
calcium ion concentration in the blood
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17-49
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Clade Clitellata
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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
Chloragogen tissue surrounds the intestine
 Synthesizes glycogen and fat
 Cells full of fat float free in the coelom as
eleocytes
 Also function in excretion
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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 with five main
trunks running lengthwise in the body
Dorsal vessel contains valves and functions as a
true heart
 Pumps blood anteriorly into 5 pairs of aortic
arches
 Aortic arches ensure steady pressure in ventral
vessel
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Clade Clitellata
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Ventral vessel serves as an aorta, delivering
blood to body walls, nephridia, and digestive tract
Blood contains colorless ameboid cells and
dissolved hemoglobin
No special gaseous exchange organs
 Gas exchanged across body surface
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Clade Clitellata
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Excretion
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Each somite, except the 1st three and terminal
one, have a pair of metanephridia
Each unit occupies parts of two adjacent somites
A ciliated funnel, the nephrostome, draws in
wastes and leads through the septum
These coil until the nephridial duct ends at a
bladder that empties outside at nephridiopore
Wastes from both the coelom and the blood
capillary beds are discharged
Aquatic oligochaetes excrete toxic ammonia
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17-54
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Clade Clitellata
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Terrestrial worms excrete less toxic urea
Chloragogen cells that break off and enter
nephridia produce urea and ammonia
Salts pass across the integument, apparently by
active transport
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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 somite contain both
sensory and motor fibers
Neurosecretory cells in brain and ganglia secrete
neurohormones
 Regulate reproduction, secondary sex
characteristics, and regeneration
One or more giant axons located in ventral nerve
cord increase rate of conduction and stimulate
contractions of muscles in many segments
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17-57
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17-58
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Clade Clitellata
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Lack eyes but have many photoreceptors in the
epidermis
Free nerve endings in tegument are probably
tactile structures
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Clade Clitellata
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General Behavior
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Avoid bright light (negative phototaxis)
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
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Monoecious
In Lumbricus, reproductive structures are located
in somites 9 through 15
Sperm produced by testes mature in seminal
vesicles and pass into sperm ducts
Eggs are discharged by ovaries into coelomic
cavity
 Ciliated funnels transport them outside
Two pairs of seminal receptacles receive and
store sperm
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17-62
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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 and chitinous band to form a cocoon
Cocoon passes forward and eggs, albumin, and
sperm are added
Fertilization and embryogenesis occur in cocoon
Young worms emerge from cocoon
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17-64
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Clade Clitellata
Class Hirudinida: Leeches
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Divided into 3 orders
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Hirudinea
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34 segments, lack setae and possess anterior and
posterior suckers
Acanthobdellidae
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Hirudinea (‘true” leeches)
Acanthobdellidae
Branchiobdellidae
27 segments, setae only present on the first five
segments, and have a posterior sucker
Branchiobdellidae
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14 or 15 segments, no setae, and an anterior sucker
Commensal or parasitic on crayfish
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Clade Clitellata
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Diversity
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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
Most are flattened
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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Clade Clitellata
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Form and Function
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Usually have a fixed number of segments
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17-68
Appear to have more due to superficial annuli
Lack distinct coelomic compartments
 No septa
In most, coelomic cavity filled with connective
tissue and spaces (lacunae)
Lacunae channels may serve as auxiliary
circulatory system
No setae
 Developed suckers for attachment
Gut specialized for storage of large quantities of
blood
Most use suckers to attach and “inchworm” along
surfaces
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17-69
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Clade Clitellata
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Nutrition
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17-70
Not all are parasites, many are predaceous
Freshwater leeches have a proboscis 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
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17-71
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Clade Clitellata
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Respiration and Excretion
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17-72
Some fish leeches have gills
All other leeches exchange gases across
epidermis
10 to 17 pairs of nephridia
Coelomocytes and other special cells may assist
in excretion
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Clade Clitellata
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Nervous and Sensory Systems
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Two “brains”
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Anterior fused ganglia form a ring around the pharynx
Seven pairs of posterior fused ganglia
21 pairs of segmental ganglia in between along a
double nerve cord
Epidermis contains free sensory nerve endings
and photoreceptor cells
Row of sensillae in central annulus of each
segment.
Pigment-cup ocelli are present
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Clade Clitellata
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Reproduction
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Hermaphroditic and practice cross-fertilization
Sperm transferred by hypodermic impregnation
Clitellum secretes cocoon to receive sperm and egg
Cocoons are buried in mud or damp soil
Development is similar to that of oligochaetes
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Clade Clitellata
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Circulation
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Coelom reduced by invasion of connective tissue
and chloragogen tissue
Forms system of coelomic sinuses and channels
Some have a typical oligochaete circulatory
system
 Coelomic system is auxiliary
Some lack blood vessels and coelomic sinuses
serve as only vascular system
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Clade Clitellata
Classification
Class Polychaeta
 Class Oligochaeta
 Class Hirudinida
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17-76
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Phylum Echiura
Diversity
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Approximately 140 species of marine worms
that burrow into mud or sand
Live in empty snail shells or sand-dollar
tests, or rocky crevices
Found in all oceans
Length varies from a few millimeters to 40 or
50 cm
17-77
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17-78
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Phylum Echiura
Form and Function
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Sausage-shaped
Inextensible proboscis anterior to the mouth
Often called spoon worms”
Simple nervous system with a ventral nerve
running length of the body
Ciliated groove on the proboscis allows them
to gather detritus over the mud while lying
buried
Muscular body wall is covered by a cuticle
and epidermis which may be smooth or
covered by papillae
17-79
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17-80
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Phylum Echiura
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Large coelom
Digestive tract long and coiled
Pair of anal sacs may serve an excretory and
osmoregulatory function
Most have a closed circulatory system with
colorless blood
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Hemoglobin found in certain cells and in coelomic
corpuscles
Respiration probably occurs in hindgut which
is continually filled and emptied by cloacal
irrigation
17-81
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17-82
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Phylum Echiura
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Sexes are separate
Gonads produced by special regions in
peritoneum in each sex
Fertilization usually external
Early cleavage and trochophore stages
similar to annelids
17-83
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Phylum Sipuncula
Diversity
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Approximately 250 species of benthic marine
worms
Sedentary, living in burrows of mud or sand,
snail shells, coral crevices, or among
vegetation
More than ½ restricted to tropical zones
Some are tiny, slender worms, but most range
from 3 to 10 cm in length
Some are known as “peanut worms” because
when disturbed, they contract to a peanut
shape
17-84
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17-85
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Phylum Sipuncula
Form and Function
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No segmentation or setae
Slender, retractable introvert or proboscis at
anterior end
Walls of the trunk are muscular
17-86
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Phylum Sipuncula
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Nutrition
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Little known about feeding habits
Some appear to be detritivores and others
suspension feeders
Some nutrition may come from dissolved organic
matter in the surrounding water
From burrow or hiding place, they extend
tentacles to explore and feed
Collected organic matter moved from mucus on
tentacles to mouth by ciliary action
Retractor muscles retracts the introvert to allow
tubular compensation sacs along the esophagus
to accept fluid from tentacles
Large fluid-filled coelom
Digestive tract is U-shaped
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17-88
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Phylum Sipuncula
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Respiration
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Lack a circulatory and respiratory system
Gas exchange appears to occur across the
introvert and tentacles
Nervous and Sensory Systems
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17-89
Bilobed cerebral ganglion behind tentacles
Ventral cord extends the length of body
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Phylum Sipuncula
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Reproduction
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Sexes are separate
Sex organs develop seasonally within the
connective tissue covering the origins of the
retractor muscles
Sex cells are released through the nephridia.
Asexual reproduction occurs by transverse
fission
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Evolutionary Significance of Metamerism
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Origins of Metamerism and the Coelom
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17-91
No satisfactory explanation for origins of
metamerism and coelom has gained acceptance
Coelom may have been advantageous as a
hydrostatic skeleton
Coelomic fluid would have acted as a circulatory
fluid and reduce need for flame cells everywhere
Coelom could store gametes for timed release
 Would require nervous and endocrine control
Unlikely that segmentation is homologous among
annelids, arthropods, and chordates
Current evidence supports the hypothesis that
segmentation arose independently multiple times
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Evolutionary Significance of Metamerism
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Selective advantage of a segmented body for
annelids appears to lie in the efficiency of
burrowing
However, does not explain segmentation in
arthropods given the rigidity of the exoskeleton
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Phylogeny and Adaptive Diversification
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Molluscs and annelids share many
developmental features so are presumed to
be closely related
However, shared features are likely to be a
retained ancestral feature for
lophotrochozoan protostomes
Pogonophorans and vestimentiferans were
once placed in Annelida but are now in clade
Siboglinidae
Molecular analyses place sipunculids and
echiurans closely related to the annelids
17-93
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Phylogeny and Adaptive Diversification
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Some biologists place echiurans within
Annelida because they have serially repeated
structures
One recently developed phylogenetic tree
place echiurans near capitellid polychaetes
because both dwell in sediments
17-94
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Phylogeny and Adaptive Diversification
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With more studies, Echiura, like Pogonophora, may
no longer be a valid phylum
Placement of Sipuncula is contentious
 Members are not metameric and lack setae
 Larval development similar to annelids, molluscs,
and echiurans
 Molecular data may clarify position within
Lophotrochozoa
 Presently, depicted as the sister taxon to a clade
of annelids and echiurans
Polychaeta is a paraphyletic group
17-95
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Phylogeny and Adaptive Diversification
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Adaptive Diversification
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17-96
Oligochaetes are constrained by terrestrial soil
environment
Polychaetes inhabit a wide range of habitats
Septal arrangement with fluid-filled compartments
has been varied for precise movements
Feeding adaptations vary widely, from chitinous
jaws to specialized tentacles
Leeches have developed both parasitic and
predatory adaptations