Phylum Platyhelminthes

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Phylum PlatyhelminthesSimple Animals

• Ch. 10

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Phylum Platyhelminthes

• Greek words
• platys meaning flat
• helmins meaning worm
• Over 20,000 species
• Size ranges from 1mm to 25m

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Characteristics

• Usually flattened dorsoventrally
• Triploblastic
• Acoelomate
• Bilateral symmetry
• Unsegmented body
• Most have incomplete gut

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Characteristics

• Somewhat cephalized
• Excretory structures
• Mostly monoecious
• Nervous system
• A pair of anterior ganglia
• Longitudinal nerve cords

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Class Turbellaria

• Mostly free-living
• Bottom dwellers in freshwater and marine
  environments
• Named for turbulence in water created by beating
  cilia
• Predators and scavengers

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Class Turbellaria

• Most less than 1cm
• Most species are dark in color (brown, gray,
  black)
• Exceptions
• Some tropical species reach 60cm
• Some are very colorful

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Epidermis

• Some epidermal cells have cilia, some have
  microvilli
• Basement membrane exist under the epidermis
• Ventral epidermis has many specialized gland
  cells
• Rhabdites secret mucous
• Adhesive glands produce chemicals for
  attachment
• Releaser glands produce chemicals to dissolve
  attachment

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Muscle

• Muscle layers are found under basement membrane
• Outer circular muscle layer
• Inner longitudinal muscle layer
• Oblique muscle tissues found between dorsal and
  ventral surfaces

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Mesoderm

• Mesoderm tissue made of parenchymal cells
• Gastrodermis secretes enzymes

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Locomotion

• Gliders - undulation
• Use cilia, microvilli, and muscles
• Lay down mucous as they move

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Digestion Nutrition

• Most are carnivores scavengers
• Some are herbivores
• Some digestive systems are simple, some highly
  branched
• Incomplete digestive system
• Mouth leads to a pharynx
• Planarians are tripartite

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Exchange with Environment

• No respiratory system
• CO2 O2 are exchanged with environment
• Wastes removed by diffusion
• Protonephridia are present
• Allows regulation of water and ions
• Use flame cells

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Nervous System

• Some have nerve net
• Some have nerve net developed into nerve cords
  and cerebral ganglia
• Planarians have
• Auricles
• Sensory lobes on head
• Ocelli
• Simple eyes detect light

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Asexual

• Many reproduce by asexual fission
• Zooids result from fission
• They regenerate missing body parts

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Sexual

• Monoecious
• Male parts
• Sperm ducts, seminal vesicle
• Penis
• Female parts
• One to many pairs of ovaries
• Oviducts, genital chamber
• Cross-fertilization between two individuals
• Fertilized eggs usually laid in cocoon

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Class Trematoda

• Flukes - parasitic flat worms
• Have complicated life cycles
• Have vertebrate host
• 1mm to 6 cm in length
• Feed on host cells
• Digestive tract has mouth and pharnyx
• Also absorb nutrients across body wall

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Examples

• Chinese liver fluke
• Chlonorchis sinensis
• Common liver fluke
• Fasciola hepatica

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Chinese Liver Fluke Life Cycle

1. Human eats infected raw fish (definitive host)
2. Adult flukes live in liver
3. Eggs pass in feces
4. Eaten by snails (intermediate host)
5. Fluke larvae leave snail and swim
6. Larvae find fish and encyst in fish muscle
   (intermediate host)

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Class Cestoidea

• Tapeworms all parasites
• Have vertebrate host
• Most specialized flat worms
• No color
• 1mm to 25 m in length
• Unique
• No mouth
• No digestive tract, absorb nutrients directly
• Most have a long series of proglottids

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• Unique
• No mouth
• No digestive tract, absorb nutrients directly
• Most have a long series of proglottids
• Each proglottid contains a complete set of
  reproductive structures

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Examples

• Beef tapeworm
• Taenia saginata
• Pork tapeworm
• Taenia solium
• Fish tapeworm
• Diphyllobothrium latum
• Dog tapeworm
• Dipylidium canium

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Beef Tapeworm Life Cycle

1. Cow eats grass contaminated with human feces
2. Feces contains proglottids
3. Larvae use hooks and bore through intestinal wall
   in to bloodstream
4. Larvae travel to muscle and encyst
5. Cysts are known as bladder worms
6. Humans eat measly beef
7. Larvae finish life cycle in human intestine
8. Mature tapeworms reproduce (80,000 eggs per
   proglottid)

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