CHAPTER 14
Flat Worms:
Phylum Platyhelminthes
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General Features
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Animals that actively seek food, shelter,
home sites, and mates require a different set
of strategies and body organization than
radially symmetrical sessile organisms
Two major evolutionary advances in phylum
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Cephalization
 Concentrating sense organs in the head region
Bilateral symmetry
 Body can be divided along only 1 plane of
symmetry to yield 2 mirror images of each other
 First phylum with Right and Left sides
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General Features
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Acoelomates
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Triploblastic
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blastopore becomes the mouth
Incomplete Gut
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Endoderm, Ectoderm, and Mesoderm
1st phyla to have 3 germ layers
Protostomes
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Typical acoelomates have only one internal
space, the digestive cavity
Without coelom (additional body cavity)
One opening
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Diagram of an Acoelomate Body Plan
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Phylum Platyhelminthes
Characteristics
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Commonly called flatworms
Vary from a millimeter to many meters in
length
Some free-living; others parasitic
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Stained
Planaria
Terrestrial
flatworm
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Phylum Platyhelminthes
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Platyhelminthes is divided into three classes:
Turbellaria (Planaria), Trematoda (flukes), and
Cestoda (tapeworm)
 All members of Trematoda (flukes) and Cestoda
(tapeworms) are parasitic
 Class Turbellaria
 Mostly free-living forms
 Most are bottom dwellers in marine or
freshwater
 Freshwater planarians
Found in streams, pools, and hot springs
Terrestrial flatworms limited to moist places
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Phylum Platyhelminthes
Form and Function
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Epidermis and Muscles
 Most have cellular, ciliated epidermis on a
basement membrane
 Most turbellarians have dual-gland
adhesive organs
Viscid gland cells fasten microvilli of anchor
cells to substrate
 Secretions of gland cells provide a quick
chemical detachment
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Cross Section Of Planaria
Turbellaria
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Releasing and Attaching (Viscid) Glands
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Phylum Platyhelminthes
Nutrition and Digestion
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Some have a mouth, pharynx, and intestine
In planarians
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Intestine has three branches
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Pharynx may extend through the ventral mouth
One anterior and two posterior
Mouth of trematodes (parasitic flukes)
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Opens near the anterior end
Pharynx is not extensible
Intestine ends blindly, varies in degree of branching
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Structure of Planarian
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Human Liver Fluke - trematode
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Phylum Platyhelminthes
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Planaria (Turbellaria)
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Carnivorous and detect food by chemoreceptors
Food trapped in mucous secretions from glands
Wrap themselves around prey
Extend the pharynx to suck up bits of food
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Phylum Platyhelminthes
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Trematodes (parasitic flukes)
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Feed on host cells, cellular debris, and body
fluids
Enzymes from the intestine are secreted for
extracellular digestion
Phagocytic cells in gastrodermis complete
digestion at intracellular level
Undigested food egested out the pharynx
Cestodes (tapeworm)
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Rely on the host’s digestive tract
Absorb digested nutrients
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Phylum Platyhelminthes
Excretion and Osmoregulation
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Flatworms have protonephridia (kidney)
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Used for osmoregulation
Beating flagella drive fluids down collecting ducts
Wall of the duct beyond the flame cell bears folds
or microvilli to resorb ions and molecules
Majority of metabolic wastes
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Removed by diffusion through body wall
Collecting ducts join and empty at nephridiopores
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Phylum Platyhelminthes
Nervous System
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Subepidermal nerve plexus resembles
nerve net of cnidarians
One to five pairs of longitudinal nerve
cords lie under the muscle layer
Freshwater planarians
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Brain is a bilobed cerebral ganglion (mass
of nerve cells) anterior to the ventral nerve
cords
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Phylum Platyhelminthes
Sense Organs
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Ocelli (light-sensitive eyespots)
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Tactile and chemoreceptive cells
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Present in turbellarians, and larval
trematodes
Abundant
Statocysts (equilibrium) and
rheoreceptors (sense direction of water
currents)
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Phylum Platyhelminthes
Reproduction and Regeneration
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Fission
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Many turbellarians constrict behind the pharynx and
separate into two animals
Each half regenerates the missing parts
 Provides for rapid population growth
Regeneration
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If the head and tail are cut off
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Each end grows the missing part; it retains polarity
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Phylum Platyhelminthes
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Nearly all are monoecious
(hermaphroditic) but cross-fertilize
Male Structures
One or more testes are connected to one
vas deferens
 The vas deferens runs to a seminal vesicle
 A nipple-like penis or extensible tentacle is
the copulatory organ
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Phylum Platyhelminthes
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Turbellarians develop male and female
organs opening at a common pore
After copulation, eggs and yolk cells
enclosed in small cocoon
Attach by a stalk to plants
 Embryos emerge and resemble little adults
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Phylum Platyhelminthes
Classification of Phylum Platyhelminthes
 Class Turbellaria - planaria
 Class Trematoda - flukes
 Class Cestoda - tapeworm
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Phylum Platyhelminthes
Class Turbellaria
Mostly free-living
 Range from 5 mm to 50 cm long
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Very small planaria swim by cilia
Others move by cilia
 Glide over a slime track secreted by adhesive
glands
 Rhythmical muscular waves pass backward
from the head
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14-25
Different Intestinal Pattern of Turbellarians
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Marine tubellarian
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Phylum Platyhelminthes
Classification of Phylum Platyhelminthes
 Class Turbellaria - planaria
 Class Trematoda - flukes
 Class Cestoda - tapeworm
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Phylum Platyhelminthes
Class Trematoda
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All trematodes are parasitic flukes
Most adults are endoparasites (inside) of vertebrates
They resemble turbellaria but the tegument (skin)
lacks cilia in adults
Sense organs are poorly developed
Adaptations for parasitism include:
 Penetration glands
 Hooks and suckers for adhesion
 Increased reproductive capacity
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Phylum Platyhelminthes
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General Trematoda Life Cycle
Egg passes from definitive host and must
reach water
 Hatches into a free-swimming ciliated
larva, the miracidium
 Miracidium penetrates tissues of a snail
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Transforms into a sporocyst
Sporocyst reproduces asexually to form
redia
 Rediae reproduce asexually and form
cercaria
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Phylum Platyhelminthes
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Cercariae emerge from the snail
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Penetrate a 2nd intermediate host (fish)
Develop into metacercariae (juvenile flukes)
Metacercaria develop into adults when eaten by
definitive host
Some serious parasites of humans and
domestic animals are trematodes
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Example: sheep live fluke, human liver fluke
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Phylum Platyhelminthes
Sheep Liver Fluke (Fasciola hepatica)
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Adult fluke lives in bile passageways in the
liver of sheep
Eggs are passed out in feces
Miracidia hatch and penetrate snails to
become sporocysts
After two generations of rediae
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Cercaria encyst on vegetation and await being
eaten by sheep
When eaten, metacercariae develop into
young flukes
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Phylum Platyhelminthes
Clonorchis sinensis: Human Liver Fluke
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Most important human liver fluke
Common in China, Japan, and Southeast Asia
Also infects cats, dogs, and pigs
Adult fluke is 10–20 mm long with an oral and
ventral sucker
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Phylum Platyhelminthes
Clonorchis Life Cycle (Liver Fluke)
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Adults live in bile passageways of humans and other
fish-eating mammals (sexual reproduction occurs
here)
Eggs containing a complete miracidium are shed
into water with feces
The eggs hatch only when ingested by snails of
specific genera
Miracidium enters snail tissue and transforms into a
sporocyst
Sporocyst produces one generation of rediae, which
begin differentiation (asexual reproduction)
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Human Liver Fluke Life Cycle
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Phylum Platyhelminthes
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Rediae pass into the snail liver
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Cercariae escape into water
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Make contact with fish
Bore into fish muscles or under scales
Shed tail and turn into a metacercariae cyst
A mammal eats raw fish
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Turns into tadpole-like cercariae
Cyst dissolves and flukes migrate up bile duct
Heavy infection can destroy the liver and
result in death
Control of parasites
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Destroy snails and thoroughly cook fish
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Phylum Platyhelminthes
Schistosoma: Blood Flukes
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Over 200 million people infested with
schistosomiasis
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Common in Africa, South America, West Indies,
and the Middle and Far East
Sexes are separate
3 species with varied location:
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large intestine
small intestine
urinary bladder
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Phylum Platyhelminthes
Schistosoma Life Cycle
 Eggs discharged in human feces or urine
 In water, eggs hatch as ciliated miracidia
 Contact with a particular species of snail to
survive
 In the snail, they transform to sporocysts
 Sporocysts produce cercaria directly
 Cercariae escape the snail and swim until
they contact bare human skin or other host
 Cercariae pierce the skin and shed their tails
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Phylum Platyhelminthes
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Enter blood vessels and migrate to the
hepatic portal blood vessels (blood vessel
from digestive tract to liver)
Develop in the liver and they migrate to
target sites
Eggs released by females are extruded
through gut or bladder lining and exit with
feces or urine
Eggs that remain behind become centers of
inflammation
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Blood Fluke
Life Cycle
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Cut Liver of individual who dies from hematemesis
(vomiting blood). 180 adult flukes were found in autopsy
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Phylum Platyhelminthes
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Control: proper disposal of human wastes
Schistosoma dermatitis (swimmer’s itch)
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Occurs when cercariae penetrate an unsuitable
host such as a human (our immune system fights
them off leading to inflammation -itch)
Normal host many be a bird or other animal
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Lung Fluke - from uncooked crab meat
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Phylum Platyhelminthes
Classification of Phylum Platyhelminthes
 Class Turbellaria - planaria
 Class Trematoda - flukes
 Class Cestoda - tapeworm
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Phylum Platyhelminthes
Class Cestoda
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Tapeworms have long flat bodies with scolex
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Holdfast structure with suckers and hooks
Scolex is followed by a linear series of
reproductive units or proglottids
Lack a digestive system
Lack sensory organs except for modified
cilia
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Tapeworm:
Scolex is site of
attachment.
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Tegument of a
Tapeworm:
Many microthriches
help increase surface
area for absorption.
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Phylum Platyhelminthes
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Tegument is has no cilia
Entire surface of cestodes is covered with
projections (microtriches) similar to
microvilli seen in the vertebrate small
intestine
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Microtriches increase the surface area for food
absorption
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Phylum Platyhelminthes
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Nearly all cestodes require two hosts
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Adult is parasitic in the digestive tract of the
vertebrate
Over 1000 species of tapeworms known,
infecting almost all vertebrates
Most tapeworms do little harm to host
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Phylum Platyhelminthes
Taenia saginata: Beef Tapeworm
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Lives as an adult in the digestive canals of humans
Juvenile form found in intermuscular tissue of cattle
Mature adults can reach over 10 meters in length
with over 2000 proglottids (segments conaining
reproductive organs)
Scolex has four suckers but no hooks
Gravid proglottids (with shelled, infective larvae)
pass in feces, single
Proglottids rupture as they dry
 Embryos are viable for five months and are picked
up by grazing
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Phylum Platyhelminthes
Beef Tapeworm Life Cycle - Continued
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Cattle swallow shelled larvae that hatch as
oncospheres
Oncospheres use hooks to burrow through
the intestinal wall into blood or lymph
vessels
When they reach voluntary muscle, they
encyst to become “bladder worms” (cyst that
resembles a bladder)
When the infected meat is eaten, the cyst
wall dissolves and the scolex evaginates to
attach to intestinal wall
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Phylum Platyhelminthes
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New proglottids develop in 2–3 weeks
Infected individuals expel numerous
proglottids daily
Infection can be avoided by eating only
thoroughly cooked beef
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Life Cycle of
Beef Tapeworm:
From human
feces, to grass, to
cattle, to meat, to
human.
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Dog Tapeworm:
Shows 3
Proglottids
(Reproductive
segments)
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Phylum Platyhelminthes
Other types of Tapeworm:
 Taenia solium: Pork Tapeworm
Diphyllobothrium latum: Fish Tapeworm
 Echinococcus granulosus:
Unilocular Hydatid
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Dog Tapeworm: humans acquire disease by unsanitary habits associated with dogs.
When eggs are ingested, larvae encyst in the liver, lungs, or other organs.
Cyst enlarges up to the size of a basketball.
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