Transcript Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 11 Annelids and Allied Taxa 17-1

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CHAPTER 11
Annelids and Allied Taxa
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Characteristics
Diversity
• Annelids exhibit segmentation or metamerism
• Evolution of metamerism allowed much greater
complexity in structure and function
– 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
• Cleavage is spiral mosaic
• 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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Phylum Annelida
Characteristics
• About 15,000 species
• 2/3 are the more obscure marine worms.
• Segmentation
– Body segments marked by circular grooves called annuli
– Metamerism
• Repetition of organs in segments called metameres or somites
– Septa separate segments
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Phylum Annelida
• Setae
– Tiny chitinous bristles called setae
– Absent in leeches
– Short setae anchor segments in earthworms
• Prevent it from slipping backward
– Long setae help aquatic worms swim
• Polychaetes
– Primarily marine and usually benthic
• Oligochaetes and leeches
– Freshwater or terrestrial soils
– Many leeches are parasites
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Phylum Annelida
Body Plan
– Head
– Terminal portion bearing the anus is the pygidium
– Head and pygidium are not considered metameres
– New metameres form in front of the pygidium
• Coelom
– Peritoneum (mesodermal epithelium) lines body wall and forms dorsal
and ventral mesenteries
• Hydrostatic Skeleton
– Except in leeches, coelom is filled with fluid and serves as a hydrostatic
skeleton
– Contraction of longitudinal muscles
• Causes body to shorten and expand
– Contraction of circular muscles
• Causes body to narrow and lengthen
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Phylum Annelida
Phylogeny
• Traditionally, annelids are divided among 3 classes
– Class Polychaeta
– Class Oligochaeta
– Class Hirudinida
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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
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
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Class Polychaeta
• 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
• Clamworm Nereis is an example of a predatory
polychaete with jaws on an eversible, muscular
pharynx
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Clade Siboglinidae (Pogonophorans)
• Formerly members of phylum Pogonophora
(beardworms)
• Discovered in 1900
Characteristics
• 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
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Clade Siboglinidae (Pogonophorans)
Internal body
• No mouth or digestive tract
• Mode of digestion puzzling
• Nutrients such as glucose and amino acids absorbed
from seawater from a mutualistic relationship with
chemoautrophic bacteria that oxidizes hydrogen
sulfide
• Trophosome, derived embryonically from midgut,
houses the bacteria
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Clade Clitellata
• 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
• 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
• Diversity
–
–
–
–
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
– Sometimes called “night crawlers”
– Burrow in moist rich soil and usually live in branched
interconnected tunnels
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• Setae
Clade Clitellata
– 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
– 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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Clade Clitellata
– Mate at night during warm, moist weather
– 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
– Young worms emerge from cocoon
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Clade Clitellata
• Class Hirudinida: leeches
– 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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Phylum Echiura
Diversity
• 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
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Phylum Sipuncula
Diversity
• 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
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Ecdysozoans
CHAPTER 12
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Diversity
• Many protostomes possess a cuticle
– Non-living outer layer secreted by epidermis
• Cuticle restricts growth and must be molted via
ecdysis
• Members of Ecdysozoa molt cuticle as they grow
• Regulation of molting achieved by the hormone
ecdysone
• This biological event used by scientists to group
ecdysozoans in the same taxa
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Phylum Nematoda: Roundworms
Diversity
• About 25,000 species are described
– As many as half a million may exist
• Found in virtually all habitats in all biomes
– Topsoil may contain billions per acre
• Nematode parasites exist in nearly all animal and
plant species
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Phylum Nematoda: Roundworms
• Free-living nematodes feed on bacteria, yeasts,
fungal hyphae, and algae
• Predatory nematodes eat rotifers, tardigrades, small
annelids, and other nematodes
• Important as food for mites, insects, larvae, and fungi
• Caenorhabditis elegans is an important model for
studies of genomics and cell development and
differentiation
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Phylum Nematoda: Roundworms
Form and Function
• Distinguishing Characteristics
– Cylindrical shape
– Nonliving cuticle
– Cuticle shed during juvenile growth stages
• One of the characteristics used for placing the nematodes in the
superphylum Ecdysozoa
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Phylum Nematoda: Roundworms
Representative Nematode Parasites
• Some are parasites of humans
– Most are tropical
• Ascaris lumbricoides
– Occurs in up to 25% of people in some areas of the
southeastern U.S.
– More than 1.27 billion affected worldwide
– A female Ascaris may lay 200,000 eggs a day, which pass
out in host’s feces
– Embryos develop into juveniles in 2 weeks
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Phylum Nematoda: Roundworms
– Survive for long periods in soil
– Host swallows embryonated eggs, juveniles hatch, and
burrow through intestinal wall
– Carried through the heart to the lungs, they break into
alveoli and are carried up to tracheae
– Coughed up and swallowed, they mature in the intestine
after two months
– They feed on intestinal contents and may block or perforate
the intestines
– Infection rates tend to be higher in children and males
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Phylum Nematoda: Roundworms
• Hookworms
– Anterior end of these small (9–11 mm) worms has a hooklike curve
– Necator americanus, most common hookworm.
– Sexes are separate
– Large plates in mouth cut into intestinal mucosa and suck
host’s blood
– Pump through more blood than they digest
• Heavy infections cause anemia
– Eggs pass out in feces and juveniles hatch in soil
– If human skin comes in contact with soil, infective juveniles
burrow through skin to blood
– Travel in blood to the lungs, are coughed up to be
swallowed, and mature in the intestine
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Phylum Nematoda: Roundworms
• Trichina Worm
– Trichinella spiralis causes a potentially lethal trichinosis
– Adult worms burrow into intestinal mucosa and females
directly produce juvenile worms
– Juveniles penetrate blood vessels and circulate throughout
the body to all tissues and spaces
– Penetrate skeletal muscle cells, redirecting gene expression
of the musculature
• Cells lose striations and becomes a nurse cells to the parasite
– When poorly cooked meat containing encysted juveniles is
eaten, worms are liberated and mature in the intestine
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Phylum Nematoda: Roundworms
– Infect humans, hogs, rats, cats and dogs
• Hogs can become infected eating uncooked scraps of infected
meat or rats
– Four other sibling species with variable distribution,
freezing resistance, etc.
– Heavy infections cause death
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Phylum Nematoda: Roundworms
• Pinworms
– Most common parasite in the U.S. but causes little disease
– Adults live in large intestine and cecum
– Females, about 12 mm long, migrate to anal region at night
and lay eggs, causing itching
– Scratching the anal region contaminates hands and
bedclothes
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Phylum Nematoda: Roundworms
• Filarial Worms
– Eight species of filarial nematodes infect humans
• Some cause serious diseases
– Wucheria bancrofti and Brugia malayi live in lymphatic
system
• Cause inflammation and blockage of the lymphatics
vessels
• Carried by mosquitos
• Elephantiasis is caused by repeated exposure
– Swelling and growth of connective tissue causes enormous
swelling of body parts
– River blindness or onchocerciasis is carried by black flies and
infects 37 million people in tropics
– Dog heartworm, Dirofilaria immitis, is carried by
mosquitoes and is the most common U.S. filarial worm
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Phylum Nematomorpha
Diversity
• “Horsehair worms” resemble coarse hairs
• Adult structures resemble those seen in nematodes:
cuticle, epidermal cords, only longitudinal muscles,
and a nervous system
• Adults are free-living in moist habitats
• Juveniles are parasites of arthropods
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Phylum Kinorhyncha
Diversity
• Kinorhynchs are usually under 1 mm long
• About 179 species are known
• Found worldwide, from intertidal areas to 6000 m
deep
• Most live in mud
• Some have been found in algae, sponges, and other
invertebrates
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Phylum Priapulida
Diversity
• 16 species of marine worms occur in colder waters
• Found from intertidal zones to deep ocean floors,
several thousand meters deep
• Some are tube dwellers and feed on detritus
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Phylum Loricifera
Diversity
• Recently discovered (1983) in spaces between grains
of marine gravel
• 11 currently described species and 80 undescribed
species
• Widely distributed
• Most species have been found in coarse sediments at
depths of 300–450 m
• One species was collected at 8000 m
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Clade Panarthropoda
• Contains Arthropoda and two allied phyla,
Onychophora and Tardigrada
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Clade Panarthropoda
Phylum Onychophora
History and Diversity
• About 70 living species of “velvet worms”
• Range from 1.4 to 15 cm in length
• Limited to moist, leafy rain forest habitat in tropical and
subtropical regions
• Changed little over 500 million years
• Most are predaceous, some live in termite nests
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Clade Panarthropoda
Phylum Tardigrada
Diversity
• Very small, less than 1 mm long
• Most of the 300–400 known species known live in a water film around
mosses and lichens
– Few are marine
• Cryptobiosis
– Terrestrial tardigrades can suspend metabolism to survive harsh
conditions
– Tardigrades can dehydrate from 85% water to only 3% water
• In this state they can resist extreme temperatures, ionizing
radiation, oxygen deficiency, etc. for years
– When water is available, they become metabolically active again
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