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CHAPTER 14
Flatworms Phylum
Platyhelminthes, Mesozoa, Nemertea
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General Features n Two major evolutionary
advances n Cephalization
n Concentrating sense organs in the head region
n Primary bilateral symmetry n Body can be divided along
only 1 plane of symmetry to yield 2 mirror images of each other
n Active, directed movement most efficient with an elongated body form with anterior (head), posterior (tail), dorsal, and ventral sides
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General Features
n Mesoderm well-defined n Triploblastic (3 germ layers – ectoderm, endoderm, mesoderm)
n Typical acoelomates have only one internal space, the digestive cavity n Region between the epidermis and digestive cavity is
filled with parenchyma
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Phylum Platyhelminthes
Characteristics n Commonly called flatworms n Vary from a millimeter to many meters in
length n Some free-living; others parasitic
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Form and Function n Epidermis and Muscles
n Most have cellular, ciliated epidermis n Rod-shaped rhabdites
n Swell and form a protective mucous sheath n Most turbellarians have dual-gland adhesive organs
n Viscid gland cells fasten microvilli of anchor cells to substrate
n Secretions of releasing gland cells provide a quick chemical detachment
Phylum Platyhelminthes
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Nutrition and Digestion n Cestodes have no digestive system n Others have a mouth, pharynx, and intestine n In planarians
n Pharynx may extend through the ventral mouth n Intestine has three branches
n One anterior and two posterior n Gastrovascular cavity lined with columnar
epithelium n Mouth of trematodes opens near the anterior
end n Pharynx is not extensible n Intestine ends blindly, varies in degree of branching
Phylum Platyhelminthes
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n Planaria n Carnivorous and detect food by chemoreceptors n Food trapped in mucous secretions from glands
and rhabdites n Wrap themselves around prey n Extend the proboscis to suck up bits of food
Phylum Platyhelminthes
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n Trematodes n Feed on host cells, cellular debris, and body
fluids n Enzymes from the intestine are secreted for
extracellular digestion n Phagocytic cells in gastrodermis complete
digestion at intracellular level n Undigested food passed out the pharynx
n Cestodes n Rely on the host’s digestive tract n Absorb digested nutrients
Phylum Platyhelminthes
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Excretion and Osmoregulation n Flatworms have protonephridia
n Used for osmoregulation (fluid/ion control) n Beating flagella drive fluids down
collecting ducts n Wall of the duct beyond the flame cell
bears folds or microvilli to resorb ions and molecules
n Collecting ducts join and empty at nephridiopores
n Majority of metabolic wastes n Removed by diffusion across the body
wall
Phylum Platyhelminthes
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Nervous System n Subepidermal nerve
plexus resembles nerve net of cnidarians
n One to five pairs of longitudinal nerve cords lie under the muscle layer
n Freshwater planarians n One ventral pair of nerve
cords forming a ladder-type pattern
n Brain is a bilobed ganglion anterior to the ventral nerve cords
Phylum Platyhelminthes
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Sense Organs n Ocelli (light-sensitive
eyespots) n Tactile and chemoreceptive
cells n Abundant, especially in
the ear-shaped auricles n Statocysts (equilibrium) and
rheoreceptors (sense direction of water currents)
n Sensory nerve endings found in n Oral suckers and genital
pores of parasitic groups
Phylum Platyhelminthes
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Reproduction and Regeneration
Class Turbellaria n Fission
n Many turbellarians constrict behind the pharynx and separate into two animals
n Each half regenerates the missing parts n Provides for rapid population growth
n Regeneration n If the head and tail are cut off
n Each end grows the missing part
Phylum Platyhelminthes
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n Nearly all are monecious but cross fertilize n Turbellarians develop male and female organs
opening at a common pore n After copulation, eggs and yolk cells enclosed
in small cocoon n Attach by a stalk to plants n Embryos emerge and resemble little adults
Phylum Platyhelminthes
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Class Trematoda Clonorchis: Human Liver Fluke n Adults live in bile passageways of humans
and other fish-eating mammals n Eggs containing a complete miracidium are
shed into water with feces n The eggs hatch only when ingested by snails
of specific genera n Miracidium enters snail tissue and
transforms into a sporocyst n Sporocyst produces one generation of
rediae, which begin differentiation
Phylum Platyhelminthes
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n Rediae pass into the snail liver n Continue embryonation into tadpole-like cercariae
n Cercariae escape into water n Bore into fish muscles or under scales n Shed tail and encyst as metacercariae
n A mammal eats raw fish n Cyst dissolves and flukes migrate up bile duct
Phylum Platyhelminthes
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Schistosoma: Blood Fluke n Eggs discharged in human feces or urine n In water, eggs hatch as ciliated miracidia n Must contact a particular species of snail to
survive n In the snail, they transform to sporocysts n Sporocysts produce cercaria directly n Cercariae escape the snail and swim until
they contact bare human skin n Cercariae pierce the skin and shed their tails
Phylum Platyhelminthes
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n Enter blood vessels and migrate to the hepatic portal blood vessels
n Develop in the liver and they migrate target sites
n Eggs released by females are extruded through gut or bladder lining and exit with feces or urine
n Eggs that remain behind become centers of inflammation
Phylum Platyhelminthes
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Class Cestoda Taenia saginata: Beef Tapeworm n Lives as an adult in the alimentary canal of
humans n Juvenile form found in inter-muscular tissue of
cattle n Mature adults can reach over 10 meters in length
with over 2000 proglottids n Scolex has four suckers but no hooks n Gravid proglottids (with shelled, infective larvae)
pass in feces
Phylum Platyhelminthes
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n Each mature proglottid has muscles and parenchyma plus male and female organs
n Gravid proglottids usually crawl out of feces n Proglottids rupture as they dry
n Embryos are viable for five months and are picked up by grazing
Phylum Platyhelminthes
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Life Cycle n Cattle swallow shelled larvae that hatch as
oncospheres n Oncospheres use hooks to burrow through
the intestinal wall into blood or lymph vessels
n When they reach voluntary muscle, they encyst to become bladder worms (cysticerci)
n When the infected meat is eaten, the cyst wall dissolves and the scolex evaginates to attach to intestinal mucosa
Phylum Platyhelminthes
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n New proglottids develop in 2–3 weeks n Infected individuals expel numerous
proglottids daily n Infection can be avoided by eating only
thoroughly cooked beef
Phylum Platyhelminthes