Download Phylum Platyhelminthes (Flatworms)

Phylum Platyhelminthes (Flatworms)
Introduction
1. Bilaterally symmetrical, acoelomate, unsegmented, free-living or parasitic worms with a mouth
& enteron (absent in some parasitic groups) but no anus.
2. Anterior brain & longitudinal nerve cords; protonephridial excretory system; complex
hermaphrodite reproductive system; cephalization.
3. Spiral cleavage development in some. Division is oblique to the egg polar axis. Egg division is
unequal into micromeres and macromeres.
4. Triploblastic: three embryonic layers; ectoderm, endoderm & mesoderm.
5. Mesoderm consists of muscle fibres & mesenchyme.
Diversity
 What is the significance of the lack of a circulatory system?
1) Limits free-living flatworms to relatively small sizes.
2) Makes the flattened shape of larger species advantageous for gaseous exchange.
 Why are platyhelminths restricted to certain environments?
3) Body surface, acting as permeable respiratory surface, is potential site of fluid loss. This
restricts platyhelminths to environments in which dehydration is unlikely to occur.
4) Aquatic and moist terrestrial habitats & as parasites in body cavities of other animals.
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A. Turbellarians (Planarians)
 What are the general features of Turbellarians?
1) Free-living (most), acoelomate worms with ciliated epidermis.
2) Most are predators or scavengers; some are true parasites.
3) Some have symbiotic relationships with other animals.
4) E.g. A rhabdocoel lives in digestive system of gastropod or bivalve. Another rhabdocoel lives
in tube feet of starfish.
5) Habitats range from marine to freshwater to moist terrestrial.
6) Dorsoventral flat with small tentacles or AURICLES on the head.
7) Dark colour but the marine ones are bright.
B. Trematodes (Flukes)
I.

Endoparasitic (most) & have 2 or 3 hosts in life cycle.

Cavity in the holdfast organs.

Dorsoventral flat bodies
Digeneans
1) Flukes with 2 or 3 hosts, the first a mollusc & the last a vertebrate.
2) Molluscan intermediate host = gastropod or bivalve.
3) Two muscular holdfast organs; anterior ORAL SUCKER (surrounds mouth) & larger ventral
sucker or ACETABULUM.
4) Adults occur in definitive host & inhabit any organ in the body.
II.
Aspidobothreans
1) Single-host life cycle.
2) Endoparasitic, ectoparasitic or ectocommensal in or on
hosts.
3) Large ventral sucker is divided by septa into alveoli.
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C. Monogenea (Flukes)

Single-host life cycle.

Posterior attachment organ (OPISTHAPTOR).

Anterior PROHAPTOR (adhesive glands & suckers).

Ectoparasitic on fish & some vertebrates; very host-specific.
D. Cestodes
 List the general features of Cestodes.
1) Endoparasitic in gut or coelomic cavity of vertebrates.
2) 2 or more hosts in life cycle.
3) Attachment organ called SCOLEX, followed by a strobilus comprising a repeated series of
body units called PROGLOTTIDS.
4) Lack alimentary tract; lack any form of intestine.
5) Possess a cercomere = posterior attachment structure bearing hooks.
I.
II.
Cestodarians

Trematode-like body form.

Unstrobilated & lack a scolex.

Endoparasites of fish & turtles.
Eucestodes (Tapeworms)
1) Ribbon-like shape
2) Anterior attachment region (scolex) & strobila of proglottids.
3) Scolex attaches to host’s intestinal mucosa; behind scolex is neck.
4) Holdfast structures on scolex vary, & include: rows of hooklets (ROSTELLUM), muscular
suckers, elongate flaps of tissue (BOTHRIA), & leaf-like outgrowths (BOTHRIDIA).
5) Each proglottid contains male & female reproductive organs.
o
Immature  at anterior end of strobila; no eggs.
o
Mature  eggs in UTERUS of posterior proglottid.
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Relationships within Platyhelminthes
 Trematodes, Monogeneans, and Cestodes are placed in a group called NEODERMATA based
on molecular biology.
 Ciliated larval epidermis is replaced by SYNCYTIAL NEODERMIS in the adult.
Structure & Function
Body Wall
 What is the structure & composition of Turbellarian body wall?
1) Body wall is a ciliated epithelium. Cilia are used in swimming or gliding locomotion.
2) Epidermis contains mucus-secreting GLAND CELLS derived from ectoderm. Cells have their
main body in mesenchyme & processes extend to body surface.
3) Mucus maintains a moist body surface, lubricating locomotion, & for capturing prey.
4) Epidermis contains rod-shaped bodies called RHABDOIDS that produce mucus when
released from body surface.
5) Secretory cells in mesenchyme, below epidermis, produce RHABDITES.
6) Unstriated muscles lie beneath epidermis & include circular, longitudinal & oblique fibres.
 Describe the parasitic (Trematoda, Monogenea, Cestoda) body wall.
1) Distinctive tegument, NEODERMIS.
2) Distal cytoplasm of tegument has outer membrane & matrix containing mitochondria and
vesicles.
3) From this layer, passing through the basal lamina, are numerous microtubule-lined
cytoplasmic connections to tegumental CYTONS.
4) Cytons are flask-shaped cell bodies which lie in the mesenchyme. They are source of
structural elements of outer tegument (spines).
5) Bands of circular, longitudinal & oblique muscles between basal lamina & mesenchyme.
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 What is the difference between Trematode and Cestode body wall?
1) Cestodes have highly specialized microvilli called MICROTRICHES that cover the entire
surface of the tegument. They increase the surface area & are an adaptation for animals
lacking the gut. Trematodes have no microvilli.
2) Neodermis in Monogenean larval stage is covered by ciliated cells arranged in regions,
separated by cytoplasmic SYNCYTIUM (discontinuous syncytium) & both layers are
nucleated in early stages. Nuclei disintegrate later on.
3) Basal lamina is then formed.
4) Stem cells formed below the lamina. These cells produce cytoplasmic processes below the
lamina, connecting to the anucleate surface cytoplasm.
5) End of the free living stage  ciliated cells are lost  continuous syncytium is formed (=
neodermis).
6) Digenean larva is ciliated (MIRACIDIUM) has ciliated epidermal cells separated by ridge
cytoplasm  this is lost at the time of entering a host (mollusc)  ridge cytoplasm spread
and more cytoplasm is formed by stem cells.
Mesenchyme
 What is the mesenchyme & where is it found?
1) Space between internal organs is filled with mesenchyme.
2) Cellular connective tissue which supports organ systems, and…
3) Functions in transport of metabolites.
4) Contains secretory cell bodies.
 What are the two main categories of cell?
Fixed mesenchymal cells & stem cells.
 What is the significance of fixed mesenchymal cells in parasites?

Contain large reserves of glycogen.
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
Large & have extensive processes which interdigitate with choroid cells (cells with fluid filled
vacuoles) or with pigmented cells which are filamentous connective tissue (collagen).

These are fibers act as skeleton and anchor points for muscle cells.
 What is the significance of a lack of carbohydrate in the host diet?
1) Depletion of glycogen in fixed mesenchymal cells in parasites.
2) Stunting of worms & reduction in numbers.
 What are the features of stem cells?
1) Two types of stem cells: neoblasts & epidermal replacement cells (derived from neoblasts).
2) Neoblasts
a) Pluripotent or totipotent
b) Large nucleus & little cytoplasm.
c) Function in regeneration following asexual reproduction or trauma.
d) Act as stem cells for the gonads.
Feeding & Digestion
 What is the significance of the lack of an anus?
Undigested food must be regurgitated through the mouth.
 What is the structure & function of the intestine?
1) Lined with monolayer of endodermal cells (gastrodermis) which carry out digestion &
absorption.
2) Junctional complexes provide continuity between gastrodermis cells & mesenchyme cells.
 Does extracellular digestion occur in Platyhelminths?
1) Partial extracellular digestion may occur, with enzymes being secreted in the pharynx or by
gastrodermal cells.
2) Semi-digested material is phagocytosed by intestinal cells, in which final digestion occurs.
 What are the feeding habits of Turbellarians?

Few feed on photosynthetic organisms such as diatoms & green algae BUT most are
predators or scavengers.
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
Symbiotic, e.g. acoel Turbellarian Roscoffensis has symbiotic relationship with green algae
Tetraselmis. As a young ingests the algae but as an adult does not feed.
 Describe the digestive system of ACOEL Turbellarians.
1) Digestive system is a membrane-bound SYNCYTIAL mass, separated from other tissues by
specialized ‘wrapping’ cells & opening to the outside via simple, ciliated pharynx.
2) Feed by ciliary action, sweeping small prey & organic particles into the mouth.
3) Syncytium forms only after ingestion of food & shed off after digestion.
 Describe the digestive system of OTHER Turbellarians.
1) Have an eversible pharynx = folded, muscular tube lying within a pharyngeal tube.
2) During feeding, the free end of the tube is everted through the mouth.
3) Have a sac-like intestine which may have three (triclad), or many (polyclad), branches.
 Describe the digestive system of Trematode redia larvae.
1) Have a muscular pharynx & an intestinal sac.
2) Able to ingest host tissue & even germinal sacs of other Trematode species.
3) Gut is lined with cytoplasmic projections to increase absorptive surface area.
 Describe the digestive system of ADULT Trematodes.
1) Similar cytoplasmic projections on intestinal epithelium
2) Diet depends on their location within the host.
a. Intestinal flukes feed on material in host’s intestine.
b. Liver flukes on bile.
c. Blood flukes on erythrocytes.
3) Food is taken in through mouth by pumping action of pharynx.
 What is the significance of the lack of intestine in Cestodes?
All nutrients must be absorbed across the tegument.
 Describe energy metabolism in parasitic helminths.
1) Use carbohydrates as their major energy substrate.
2) Free-living stages do not feed, & depend on energy stores such as glycogen acquired in
previous host.
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Osmoregulation
 What is the importance of osmoregulation?
Allows animals to maintain a relatively constant balance of salts, ions & water in their tissues.
 How do freshwater animals maintain osmoregulation?
Conserve salts & get rid of excess water (hypertonic to environment).
 How do marine animals maintain osmoregulation?
1) Conserve water & eliminate salts.
2) Allow their tissues to have same ionic concentration as their environment.
 What are osmoconformers?

Animals which allow osmotic potential of their tissues to match that of the environment
(cannot regulate osmotic potential).

Parasitic Trematodes & Cestodes face similar problems in host intestine.
 What are osmoregulators?
Animals which maintain osmotic potential which differs from that of their environment.
 Describe the osmoregulatory system of platyhelminths.
1) Network of tubules running through mesenchyme, beginning in FLAME CELLS & opening to
environment via NEPHRIDIOPORES.
2) Flame cells & their associated network of tubules are called PROTONEPHRIDIUM.
3) Terminal cell of protonephridium (CAP CELL) encloses a number of cilia. Slow beating of cilia
resembles a flame.
4) Between tubule wall & cilia there is a ring of microvilli that act as valves controlling fluid influx.
5) Movement of fluid along tubule in response to beating of cilia lowers pressure in tubule,
causing fluid to enter through thin-wall region of cap cell membrane. Selective reabsorption of
ions results in production of urine hypotonic to internal body fluids.
Cilia beat  low pressure in tubules  fluid enter through the thinner parts of
the tubule wall in the region of the cap cells  selective reabsorption of ions 
urine produced (hypotonic to the internal body fluid).
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 What is the major role of the osmoregulatory system in parasitic platyhelminths?
Excretion of waste products of metabolism, e.g. (in adult digeneans) ammonia, urea, amino acids
& polypeptides.
 What enables freshwater planarians to prevent the influx of excess water?
Have many protonephridia & nephridiopores.
 How does the excretory system of Trematodes differ from that of Turbellarians?

Proximal collecting ducts merge & join two main collecting ducts which, in turn, empty into
excretory bladder.

Digenean flukes have one posterior excretory pore. Monogeneans have two anterior pores.
 Describe the excretory system of Cestoda.

Cestodes have protonephridia scattered throughout the body, draining into pairs of dorsolateral & ventro-lateral collecting vessels which open to the exterior on the posterior margin
of the terminal proglottid.

Tapeworms are osmoconformers. Main function of protonephridial system is in excretion.
 How do different stages of parasitic life history respond to totally different environments?
1) Schistosome cercaria, swimming in fresh water, must regulate its osmolarity by actively
excreting excess water.
2) As soon as it penetrates skin of mammalian host & enters blood vessel, the larva enters
environment in which it can become an osmoconformers.
3) Miracidia in eggs laid by adult female within host’s mesenteric veins must quickly become
osmoregulators when released into fresh water.
4) If successful in finding appropriate snail host, the intramolluscan stages conform to
osmolarity of their new environment.
Schistosome cercaria  swim in fresh water  excrete excess water  regulate its
osmolarity  penetrate the skin of a mammal  in the blood  becomes
osmoconformers.
Miracidia in the eggs inside blood vessels  released in fresh water (osmoregulators)
 enter an intermediate host  osmoconformers in the snail body.
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Nervous System & Sense Organs
 Describe the nervous system of ACOEL Turbellarians.

Resembles that of cnidarians.

Network of nervous tissue lying beneath surface epithelium
but outside the muscles.
 Describe the nervous system of OTHER Turbellarians.

Principal nervous system in mesenchyme, beneath subepidermal musculature.

Consists of cerebral ganglia connected by broad commissure, & three sets of longitudinal
cords (dorsal, lateral & ventral) linked by transverse connections arranged like a ladder.
 Describe the nervous system of more COMPLEX Turbellarians.
1) Elements of nervous system separated into motor & sensory pathways.
2) Sensory nerves radiate from cerebral ganglia to sense organs of the head.
3) These include tactile receptors, chemoreceptors, photoreceptors, rheoreceptors (detect
water movements over body surface) & statocysts (balance organs).
 Briefly describe the nervous system of digenean Trematodes.
Similar in layout to that of most Turbellarians.
 Briefly describe the nervous system of Cestodes.

Main concentration of nervous tissue is in scolex.

Several nerves arise from ring of ganglia.
 What is the function of the Nervous System?
1) Transmitting information via nerve impulses.
2) Secretes neuropeptides which can act at sites remote from point of secretion.
3) Secretory cells in cerebral ganglion or in main nerve cords have a role in motility,
reproduction & morphogenesis.
4) Neurosecretion has role in hormone-like control of
processes e.g. growth & development.
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Reproduction & Development
Asexual Reproduction in Turbellarians
 How do freshwater & terrestrial Turbellarians reproduce?
1) Both may reproduce asexually by transverse fission (and regeneration).
a. Freshwater triclads (Dugesia) divide behind the pharynx.
b. Original anterior end  regenerates a tail  complete individual.
c. Posterior end  grows a new head  complete individual.
2) Asexual reproduction of Trematodes & Cestodes follows sexual reproduction (see later).
Reproductive Systems
 How is the reproductive system of most platyhelminths?
Most are hermaphroditic but normally cross-fertilise.
 How does the complexity of the reproductive system vary?
1) Acoel turbellarians lack specific gonads & oviducts, the spermatogonia & oogonia lying free
in the mesenchyme.
2) Other turbellarians have well-developed gonads linked to complex copulatory organs by a
series of ducts modified for diverse functions.
a) There are more testes than ovaries.
b) Each testis is linked via a fine VAS EFFERENS to a common VAS DEFERENS,
which expands to form SPERMIDUCTAL VESICLE in which fresh sperm are stored.
c) Sperm move to copulatory complex that includes SEMINAL VESICLE & associated
prostatic glands, & copulatory organ itself (cirrus or penis).
d) Prostatic glands supply seminal fluid into seminal vesicle.
Sperms  move  to copulatory complex ( = seminal vesicle, prostatic glands
(supply seminal fluid) )and copulatory organs (cirrus or penis).
 What are the two structures of the female gonad?
1) Germarium: Produces ova (eggs).
2) Vitellarium: Produces yolk cells.
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 Define the following terms:
1) Ectolecithal: Ova associated with external yolk cells.
2) Entolecithal: Ova synthesize yolk within ovum.

Found in most animals, including acoels & polyclads.
3) Germovitellarium: Combined organ producing both ova & yolk cells.
 Which classes of platyhelminths have separate vitellaria?

Most turbellarians & flukes.

Vitellaria occur as numerous scattered follicles linked by separate vitelline ducts to main
collecting vessel.
 Which class of platyhelminths has a single vitellarium?

Many tapeworms have single vitellarium in reproductive system of each proglottid.
 Where are foreign sperm store?

Foreign sperm are stored in a copulatory BURSA or seminal receptacle (simple expansion of
oviduct or separate organ).
 What is Laurer’s canal?

An additional duct in many digenean flukes & some turbellarians which arises from oviduct &
opens on dorsal body surface.

Fertilised ova & vitelline cells pass into OOTYPE and egg shell is formed.
 How is the structure of the female tract adapted to the animal’s reproductive strategy?
1) Free-living turbellarians expend relatively less energy producing small numbers of eggs which
are deposited in cocoons.
2) Parasitic helminths (tapeworms or intestinal flukes) produce large numbers of eggs.
a) 80% of energy intake is devoted to producing eggs.
b) Eggs must escape from host to external environment.
c) Larva must find specific intermediate host.
d) High risk & low survival rates.
3) Compare cestodes with flukes.
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Parasitic worms: produce large number of eggs  eggs must escape 
intermediate host  high risk  80% of energy is devoted to producing eggs.
 What are the differences in anatomy of reproductive system of free-living & parasitic?

Distal oviduct is short in turbellarians.

In cestodes & digenean flukes, distal oviduct is long, sometimes branched, & contains large
number of eggs.
 What is advantage of strobilated cestodes over free-living planarians & parasitic flukes?
1) The single ootype in reproductive system of free-living planarians & parasitic flukes can
process only one egg at a time.
2) Trematodes such as Fasciola hepatica can produce 25,000 eggs/ day.
3) Tapeworm with several thousand proglottids can produce eggs simultaneously; over a million
eggs per worm per day.
 Define the following terms:
1) Female & male RS may not reach maturity at the same time.
2) Protandric: Testes mature first, e.g. Cestoda.
3) Protogynous: Ovaries mature first.
 Are all platyhelminths hermaphroditic?
1) No; some are gonochoristic (separate sexes).
2) Schistosomes are digenean flukes that live in blood of vertebrate hosts.
3) Male schistosome has broad, flat body with folded edges that form ventral groove.
4) Thread-like female is held in ventral groove (gynecophoric canal).
Mating (3 Types)
 Does cross-fertilization occur in platyhelminths? How?
1) Despite being hermaphroditic, most platyhelminths mate by cross fertilization.
2) Free-living worms pair with the male GONOPORE of one aligned with the female pore of
its mate.
3) Each worm’s copulatory organ is everted by hydrostatic pressure & inserted into female
atrium of the mate, where sperm are deposited.
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 How does self-insemination occur?
1) Usual method of fertilisation in some species of Cestoda.
2) Cirrus inserted into vagina of same proglottid.
 How does hypodermic impregnation occur?
1) In some turbellarians, monogeneans, and cestodes.
2) Male copulatory organ pierces body wall of the mate & sperm are released into
mesenchyme, through which they migrate to fertilise the ova.
hypodermic impregnation: male copulatory organ pierce the body wall of female
 sperm released into the mesenchyme  migrate  to fertilize the eggs  in
some turbellarians.
Development
Turbellarian development
 Describe the embryonic development of Turbellarians.
1) Embryonic development is direct, producing complete but minute worms.
2) Some species produce larva.
Trematode development
 Describe the embryonic development of Trematodes.
1) Trematode egg develops & hatches as a ciliated larva, the MIRACIDIUM.
2) Miracidium infects molluscan intermediate host.
3) Miracidia released from eggs in water & swim until they find a suitable mollusc.
4) Egg containing miracidium may be eaten by snail & larva is released inside host.
5) At time of infection of mollusc, ciliated epithelium is shed & syncytial neodermis formed.
6) Sequence of developmental stages in mollusc:
a) Miracidium transforms into SPOROCYST (sac-like body with no gut).
b) Within sporocyst, a radial generation proliferates.
c) CERCARIAE (tailed larvae infective to the next host) develop in REDIAE.
d) Rediae are sac-like with mouth, muscular pharynx & blind-ending gut.
e) Cercariae escape from sporocysts or rediae & infect the next host.
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Egg  ciliated larva (miracidium)  infect a mollusc  released  swim in water 
Egg  eaten by a snail  larva is released inside the host  ciliated epithelium is shed
 syncytial neodermis  sporocyst (a sac-like with no gut live in the rectal gland of the
snail)  redia (sac-like have a gut, mouth, pharynx)  cercaria (swim by tail)  infect
the next host (fish)  metacercariae  lose tail  cyst  adult in the intestine of a
bird or cat. This life style is similar to the human liver flukes.
 What happens if external environmental conditions are unfavorable for cercariae survival?

Rediae of trematodes such as sheep liver fluke, Fasciola hepatica, may generate more
rediae instead of producing cercariae.
Evasion of molluscan defences by trematode larvae
 How does the molluscan host defend itself against trematode larvae?

Ability to recognise foreign material as non-self & mount both cellular and humoral responses
which kill & remove invader.

Trematodes are able to avoid these defences provided they are in their normal host.
Monogenean development
 What are the ciliated larvae of Monogeneans called?

Oncomiracidia have well-developed OPISTHAPTOR used in attaching to new host.

Transformation from ciliated larval epidermis to syncytial neodermis.

Simple metamorphosis to adult.
Ciliated larvae (oncomiracidium with well developed opisthaptor for attachment) 
attach to a new host  syncytial neodermis  metamorphose into adult  release
eggs when the fish is not active
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Cestode development
 Describe the stages of Cestode development.
1) Initial infection of intermediate hosts via ingestion of egg which contains oncosphere.
2) Sometimes larva escapes from egg as free-swimming, ciliated CORACIDIUM & is eaten by
invertebrate.
3) Next larval stage is six-hooked (HEXACANTH) larva, which migrates to site amongst
viscera of invertebrate or vertebrate host.
4) Hexacanth larva transforms into METACESTODE, which grows into adult tapeworm –
sometimes in same host, but usually following one or more transfers to new hosts.
Eggs  larva (oncosphere larva) escape from eggs as Coracidium  eaten by an
invertebrate  six hooked larva (hexacanth) migrate to the viscera of the host 
metacestode  grow  adult.
 What are the larval stages in Cestode species?
1) Cestode species have a great variety of larval stages, including asexual reproduction.
2) Example: Hydatid cyst is larval stage of dog tapeworm Echinococcus.
3) Intermediate host is usually a herbivore (e.g. sheep), or humans.
4) Oncosphere larva released from egg in small intestine; passes through intestinal wall.
5) Carried in blood stream to the liver where it settles & begins to grow.
6) Cyst is fluid-filled sphere, lined internally with a germinal epithelium.
7) Germinal layer buds off daughter cysts into cavity of mother cyst.
8) Within these cysts, germinal epithelium produces PROTOSCOLICES.
9) Each protoscolex can mature into adult tapeworm if eaten by a dog (definitive host).
10) Each protoscolex can develop into another hydatid cyst if released into body cavity of
intermediate host.
Larval stage of the dog tape worm called Echinococcus has hydatid larva  enter the
intermediate host  oncosphere larva is released  into small intestine  to the blood
through the wall of intestine  attack liver  grow
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Cyst is a sphere filled with liquid and lined with germinal epithelium  budding cysts
(protoscolices are produced by the germinal epithelium inside the cysts)  infective 
if eaten by a dog (final host)  mature into adult. If not then grow into another hydatid
cyst (another intermediate host).
Cestodarian
Larvae are called lycophores  penetrate the exoskeleton of crayfish.

Significance in Agriculture & Medicine
Turbellaria & Trematodes
 What is the significance of Turbellaria & Trematoda in agriculture & medicine?
1) Turbellaria is a pest that feeds on earthworms (which improve soil conditions).
2) Trematodes: Sheep fluke, Fasciola hepatica, is major pathogen of domesticated sheep &
cattle.
1) Cercariae of sheep fluke encyst on vegetation & ingested by humans.
2) Young worms migrate through liver tissue to bile ducts, causing damage.
3) Adult flukes reside in gall bladder.
4) Fascioliasis in sheep & cattle causes reduced weight gain & low milk yield.
5) Infected livers are rejected at abattoirs resulting in economic loss to producers.
Trematode cercaria of sheep flukes encyst on vegetation and ingested by humans
 in liver  to the bile  gall bladder  damage
Infected liver are rejected  economic loss
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3) Trematodes: Blood flukes (schistosomes) infect humans, especially young children.
1) Two forms of infection: intestinal or urinary schistosomiasis.
2) Cercariae released from snail hosts penetrate skin.
3) Migrate via heart & lungs to liver, where they mature into adult worms.
4) Move to mesenteric veins of intestine & veins surrounding urinary bladder.
5) Illness results from reaction by human host to eggs which become trapped in tissues
instead of escaping to environment in feces or urine.
6) As more eggs become trapped, fibrous tissue replaces the normal structures.
7) Functions are severely affected.
Cercaria released from snail  penetrate skin  migrate via heart and lungs to 
liver  mature  move to mesenteric veins and veins of the urinary bladder 
illness because of the reaction to accumulated eggs in tissues  fibrous tissue 
damage in the function.
Cestodes
 What is the significance of Cestodes in agriculture & medicine?
1) Important as agents of human disease & of economic loss in animal husbandry.
2) Especially Echinococcus & Taenia.
3) Pork tapeworm, Taenia solium, is parasite of human intestinal tract.
1) Larval stage (CYSTICERCUS) IN PIGS.
2) Humans pass infection to pigs via egg-carrying proglottids voided with feces.
3) Humans become infected by eating pork containing cysticerci.
4) Humans can acquire hydatid infections; hydatid cysts found in any tissue.
5) Cyst rupture can lead to death.
Taenia solium a parasite of human: the larva is called cysticercus eaten and out
through human feces  passed to pigs and back to human. Hydatids grow in any
tissue and when cyst rapture  death occurs.
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Vaccination against helminths parasites
 What is the significance of Turbellaria & Trematoda in agriculture & medicine?
1) For human schistosomiasis: currently under development.
2) Against cestodes: recombinant antigen vaccines available.
Classification
Uncertain class  Acoel
Syncytial digestive tissue lacking intestinal
lumen, protonephridia absent.
1- Class Turbellaria
Free living (some commensal or parasitic),
ciliated surface epithelium.
Parasitic with ventral sucker.
2- Class Trematodes
Subclass Digenea (two or more hosts)
Subclass Aspidobotherea (single host)
3- Class Monogeneans
Parasitic with posterior attachment organ
(opisthaptor) & single host in life cycle.
Parasitic flatworms without alimentary canal.
4- Class Cestoda
Subclass Cestodaria (resemble flukes)
Subclass Eucestoda (strobilated with scolex)
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