1. Flat Worms: Phylum Platyhelminthes
CHAPTER 14
Flat Worms:
Phylum Platyhelminthes

2. Two major evolutionary advances in phylum
General Features
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
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

3. General Features Acoelomates Triploblastic Protostomes Incomplete Gut
Typical acoelomates have only one internal space, the digestive cavity
Without coelom (additional body cavity)
Triploblastic
Endoderm, Ectoderm, and Mesoderm
1st phyla to have 3 germ layers
Protostomes
blastopore becomes the mouth
Incomplete Gut
One opening

4. Diagram of an Acoelomate Body Plan

5. Phylum Platyhelminthes
Characteristics
Commonly called flatworms
Vary from a millimeter to many meters in length
Some free-living; others parasitic

6. Stained Planaria
Terrestrial
flatworm

7. Phylum Platyhelminthes
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

9. Phylum Platyhelminthes
Form and Function
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

10. Cross Section Of Planaria
Turbellaria

11. Releasing and Attaching (Viscid) Glands

12. Phylum Platyhelminthes
Nutrition and Digestion
Some have a mouth, pharynx, and intestine
In planarians
Pharynx may extend through the ventral mouth
Intestine has three branches
One anterior and two posterior
Mouth of trematodes (parasitic flukes)
Opens near the anterior end
Pharynx is not extensible
Intestine ends blindly, varies in degree of branching

13. Structure of Planarian

14. Human Liver Fluke - trematode

15. Phylum Platyhelminthes
Planaria (Turbellaria)
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

16. Phylum Platyhelminthes
Trematodes (parasitic flukes)
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)
Rely on the host’s digestive tract
Absorb digested nutrients

17. Phylum Platyhelminthes
Excretion and Osmoregulation
Flatworms have protonephridia (kidney)
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
Removed by diffusion through body wall
Collecting ducts join and empty at nephridiopores

18. Phylum Platyhelminthes
Nervous System
Subepidermal nerve plexus resembles nerve net of cnidarians
One to five pairs of longitudinal nerve cords lie under the muscle layer
Freshwater planarians
Brain is a bilobed cerebral ganglion (mass of nerve cells) anterior to the ventral nerve cords

19. Phylum Platyhelminthes
Sense Organs
Ocelli (light-sensitive eyespots)
Present in turbellarians, and larval trematodes
Tactile and chemoreceptive cells
Abundant
Statocysts (equilibrium) and rheoreceptors (sense direction of water currents)

20. Phylum Platyhelminthes
Reproduction and Regeneration
Fission
Many turbellarians constrict behind the pharynx and separate into two animals
Each half regenerates the missing parts
Provides for rapid population growth
Regeneration
If the head and tail are cut off
Each end grows the missing part; it retains polarity

21. Phylum Platyhelminthes
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

22. Phylum Platyhelminthes
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

23. Phylum Platyhelminthes
Classification of Phylum Platyhelminthes
Class Turbellaria - planaria
Class Trematoda - flukes
Class Cestoda - tapeworm
14-23

24. Phylum Platyhelminthes
Class Turbellaria
Mostly free-living
Range from 5 mm to 50 cm long
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

25. Different Intestinal Pattern of Turbellarians

26. Marine tubellarian

27. Phylum Platyhelminthes
Classification of Phylum Platyhelminthes
Class Turbellaria - planaria
Class Trematoda - flukes
Class Cestoda - tapeworm
14-27

28. Phylum Platyhelminthes
Class Trematoda
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

29. Phylum Platyhelminthes
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
Transforms into a sporocyst
Sporocyst reproduces asexually to form redia
Rediae reproduce asexually and form cercaria

30. Phylum Platyhelminthes
Cercariae emerge from the snail
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
Example: sheep live fluke, human liver fluke

31. Phylum Platyhelminthes
Sheep Liver Fluke (Fasciola hepatica)
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
Cercaria encyst on vegetation and await being eaten by sheep
When eaten, metacercariae develop into young flukes

32. Phylum Platyhelminthes
Clonorchis sinensis: Human Liver Fluke
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

33. Phylum Platyhelminthes
Clonorchis Life Cycle (Liver Fluke)
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)

34. Human Liver Fluke Life Cycle

35. Phylum Platyhelminthes
Rediae pass into the snail liver
Turns into tadpole-like cercariae
Cercariae escape into water
Make contact with fish
Bore into fish muscles or under scales
Shed tail and turn into a metacercariae cyst
A mammal eats raw fish
Cyst dissolves and flukes migrate up bile duct
Heavy infection can destroy the liver and result in death
Control of parasites
Destroy snails and thoroughly cook fish

36. Phylum Platyhelminthes
Schistosoma: Blood Flukes
Over 200 million people infested with schistosomiasis
Common in Africa, South America, West Indies, and the Middle and Far East
Sexes are separate
3 species with varied location:
large intestine
small intestine
urinary bladder

37. 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

38. Phylum Platyhelminthes
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

39. Blood Fluke
Life Cycle

40. Cut Liver of individual who dies from hematemesis
(vomiting blood). 180 adult flukes were found in autopsy

41. Phylum Platyhelminthes
Control: proper disposal of human wastes
Schistosoma dermatitis (swimmer’s itch)
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

42. Lung Fluke - from uncooked crab meat

43. Phylum Platyhelminthes
Classification of Phylum Platyhelminthes
Class Turbellaria - planaria
Class Trematoda - flukes
Class Cestoda - tapeworm

44. Phylum Platyhelminthes
Class Cestoda
Tapeworms have long flat bodies with scolex
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

45. Tapeworm:
Scolex is site of
attachment.

46. Tegument of a
Tapeworm:
Many microthriches
help increase surface
area for absorption.

47. Phylum Platyhelminthes
Tegument is has no cilia
Entire surface of cestodes is covered with projections (microtriches) similar to microvilli seen in the vertebrate small intestine
Microtriches increase the surface area for food absorption

48. Phylum Platyhelminthes
Nearly all cestodes require two hosts
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

49. Phylum Platyhelminthes
Taenia saginata: Beef Tapeworm
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

50. Phylum Platyhelminthes
Beef Tapeworm Life Cycle - Continued
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

51. Phylum Platyhelminthes
New proglottids develop in 2–3 weeks
Infected individuals expel numerous proglottids daily
Infection can be avoided by eating only thoroughly cooked beef

52. Life Cycle of Beef Tapeworm:
From human feces, to grass, to cattle, to meat, to human.

53. Dog Tapeworm:
Shows 3
Proglottids
(Reproductive
segments)

54. Phylum Platyhelminthes
Other types of Tapeworm:
Taenia solium: Pork Tapeworm
Diphyllobothrium latum: Fish Tapeworm
Echinococcus granulosus:
Unilocular Hydatid

55. Cyst enlarges up to the size of a basketball.
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.