Platyhelminthes

Phylum Platyhelminythes
Platyhelminthes
~ 20,000 extant species
 Parasitic + free-living
 Unsegmented flatworms
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Platyhelminthes
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Triploblastic, acoelomate, bilaterally
symmetrical
Incomplete gut; absent in some parasitic forms
Cephalization of nervous system
Protonephridia: excretion and osmoregulation
Hermaphroditic
Support
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Hydrostatic skeleton
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Elastic body wall
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Body musculature
Taxonomy
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Class Turbellaria
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Class Monogenea
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Class Trematoda
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Class Cestoda
Class Turbellaria
– Free-living flatworms
– Most are aquatic
– Epidermis cellular and ciliated
Feeding and digestion in
Turbellaria
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Consume invertebrates (few herbivores and
omnivores)
Locate food via chemoreception
A few are symbiotic
Turbellaria digestive system
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Mouth, pharynx, intestine = incomplete gut
Pharyngeal glands produce mucus and
proteolytic enzymes
Digestion extracellular, then phagocytization
in intestine
Turbellaria nervous system
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Sense organs
– Tactile receptors cover body concentrated anteriorly
– Chemoreception = location of food
– Statocysts = gravity detection and
orientation
– Photoreceptors
Inverted pigment cup ocelli
 Negative phototaxis
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Turbellaria nervous system
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Variable: simple net-like to cephalized & bilateral
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Ladder-like NS = more recently evolved
Class Monogenea
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Monogenetic flukes (life cycle = one host)
– Body covered by tegument
– Oral sucker reduced or absent
– Ectoparasitic (usually fish)
Class Monogenea
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Monogenetic flukes (life cycle = one host)
– Eggs hatch into ciliated larvae = oncomiracidia
– Mature and find host
Class Trematoda
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Digenetic flukes (multiple hosts)
– Body with tegument
– One or more suckers present
– Internal parasite
Fluke Digestive System
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Feed on host tissues and fluids (muscular
pharynx)
– Or, material in host gut
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One-way digestive tract: mouth, muscular
pharynx, short esophagus, intestinal cecae
Fluke Nervous System
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Ladder-like
Cerebral ganglion
Suckers with tactile receptors (bristles
and spines)
Sexual repro flukes
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Hermaphroditic
Mutual cross fertilization
Male structures
– Variable testes
Monogenetic = many
 Digenetic = two
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– Sperm-to sperm duct, copulatory
apparatus, eversible cirrus
Sexual repro flukes
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Female Structures
– Ovary to oviduct to ootype
– Oviduct joined by vitelline duct
– Seminal receptacle = blind pouch off of
oviduct
– Single uterus sometimes modified as vagina
near female gonopore
Fluke reproduction
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Mutual cross-fertilization
Sperm stored in seminal receptacle
Eggs - oviduct to ootype then fertilized
r-selected strategy (high fecundity)
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a – acetabulum
d - vitelline ducts
f - vitelline follicles
o - oral sucker
oe – oesophagus
oo – ootype
ov – ovary
ph – pharynx
sr - seminal receptable
t – testis
u - uterus
Fluke life-cycles
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Monogenetic
– One host
– Mostly external parasites of fish
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Digenetic
– Two or more hosts
– Mostly internal parasites
Fluke life-cycles:
Chinese liver fluke
Fluke life-cycles
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Digenetic Fasciola = sheep liver fluke
– Multiple hosts
– Internal parasite of vertebrates
– Intermediate host usually gastropod
Fluke life-cycles
Schistosoma mansoni
Schistosomiasis = disease with
problems from egg production, fevers,
eggs lodged in various tissues
Schistosomiasis
Schistosoma spp. cause
swimmer’s itch
Class Cestoda
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Tapeworms
– Internal parasite
– Body with tegument
– Body with anterior scolex, short neck and
proglottids
– No digestive system
Tapeworms = cestodes
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Locomotion
– Sedentary: adult on host intestinal wall
– Capable of muscular undulations
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Attachment
– Scolex
– Anterior with hooks or adhesive pad
Tapeworm digestion
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No mouth, no digestive tract
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Nutrients absorbed across tegument
Tapeworm Nervous System
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Cerebral ganglion; nerve ring in scolex
Each proglottid has additional ganglia;
connect to longitudinal nerve cords
Sensory organs reduced, tactile receptors in
scolex
Sexual Repro: tapeworms
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Hermaphroditic
Mutual cross-fertilization
Self-fertilization in some
Proglottids
– Numerous testes along margins
– Collecting tubules to coiled sperm duct
– Vas deferens to genital pore
Proglottids
– Two ovaries
– Uterus = blind sac
Tapeworm sex and fertilization
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Cirrus of each mate inserted into genital pores
Sperm stored, eggs fertilized in oviduct
Capsule material and yolk cells stored in uterus
When mature, proglottids break free
Beef Tapeworm Life Cycle
Pork Tapeworm Life Cycle