Flatworms Phylum Platyhelminthes, Mesozoa,...

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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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