Phylum Platyhelminthes

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Phylum Platyhelminthes
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• General Characteristics
• They exhibit bilateral symmetry anterior and
  posterior ends are different so are the dorsal
  (top) and ventral (bottom) surfaces
• The platyhelminths also exhibit some degree of
  cephalization Commonly referred to as the
  'flatworms' because their bodies are
  dorsoventrally flattened.
• They are acoelomates
• This phylum (and all remaining phyla) possess 3
  germ layers (triploblastic)
• The mesoderm (third germ layer) gives rise to
  muscles, various organ systems, and the
  parenchyma, a form of solid tissue containing
  cells and fibers

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• Outer Body Covering
• The body of some platyhelminthes (e.g.,
  turbellarians) is covered by a ciliated epidermis
• Epidermal cells contain rod-shaped structures
  called rhabdites that when released into the
  surrounding water, expand and form a protective
  mucous coat around the animal

• The outer body covering of other platyhelminthes
  (e.g., parasitic forms) is a non-ciliated
  tegument
• The tegument is referred to as a syncytial
  epithelium

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• Organ Systems of the Platyhelminthes
• Digestive System
• Some of the flatworms possess a digestive
  system, with a mouth, pharynx, and a branching
  intestine from which the nutrients are absorbed
• The intestine, with only one opening, is a blind
  system

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Organ Systems of the Platyhelminthes cont.

• Excretory System (osmoregulation)
• A network of water collecting tubules adjacent
  to flame cells or a protonephridia
• When cilia beat they move water into the tubules
  and out the body through pores called
  nephridiopores

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• Organ Systems of the Platyhelminthes cont
• Muscular System
• Below the epidermis are layers of circular and
  longitudinal muscle fibers used in locomotion

• Nervous System
• Includes anterior cerebral ganglia,
  longitudinal nerve cords, and some lateral nerves
• Most free living planarians and parasitic larval
  forms possess a variety of sensory organs (e.g.,
  eye spots, statocysts, rheoreceptors)

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Organ Systems of the Platyhelminthes cont.

• Reproductive System
• Most are capable of some form of asexual
  reproduction (e.g., many turbellarians reproduce
  by fission)
• Most flatworms are hermaphroditic however, they
  often pair with other individuals to exchange
  gametes

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Platyhelminthes Taxonomy
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• Class Turbellaria
• Free-living flatworms mostly marine organisms
• Range in size from microscopic (interstitial
  species between sand grains) to extremely large
  (two feet)

• Locomotion
• Most move by means of cilia and mucous
• Muscle contractions also permit turning,
  twisting and folding of the body

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• Class Turbellaria cont
• Nutrition
• Turbellarians are carnivores and prey on other
  animals or eat dead animal remains.
• Planarians have a muscular pharynx that they can
  insert into their prey and then pump to bring in
  food fragments
• These animals have a highly divided gut to
  greatly increase the surface area for digestion
  and absorption
• Senses
• They have well developed sensory structures,
  including eyespots, mechanoreceptors, and
  chemoreceptors

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• Class Turbellaria cont
• Reproduction
• Planarians are capable of asexual reproduction
  via fission
• Also capable of regeneration exhibit both
  anterior- posterior and lateral polarity
• They are hermaphrodites but usually exhibit
  cross-fertilization
• The penis of some turbellarians is modified as a
  hollow stylet sperm tranfer is by hypodermic
  impregnation, in which the copulating partners
  stab each other and inject sperm

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• Class Trematoda
• Flukes that live as parasites either on or in
  other organisms.
• Outer body lacks cilia tegument has a layer of
  glycoproteins that are important in protection
  and absorption
• Possess 2 suckers
• 1. Oral sucker which attaches to organs of the
  host
• 2. Ventral sucker or acetabulum used to attach
  to host tissues

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• Types of Hosts
• Often have complex life cycles that alternate
  between sexual and asexual stages.
• Most require at least 2 different kinds of hosts
  to complete their life cycle
• 1. Definitive host (primary host)
• The host in which the parasite matures and
  reproduces (sexually)
• The host in which eggs are released
• 2. Intermediate host
• Hosts in which larval stages develop and undergo
  asexual reproduction
• Results in an increase in the number of the
  individuals

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• General Life Cycle - Chinese liver fluke,
  Clonorchis sinensis
• Adults live in the bile ducts of humans, dogs,
  and cats
• There are 2 intermediate hosts a snail and a
  fish
• Eggs are passed out of the definitive host and
  hatch as ciliated larvae called miracidia
• The miracidia penetrates a snail molluscan host
  and becomes a sporocyst
• They undergo asexual reproduction producing
  larvae called rediae
• Rediae often asexually produce more rediae, but
  will eventually give rise to larvae called
  cercariae
• They leave the molluscan host and penetrate fish
  
• They encyst in the fish tissues as the
  metacercaria
• Consumption of infected fish results in the
  metacercaria excysting in the gut and migrating
  to the bile duct

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• Schistosoma
• Schistosoma spp. is a common blood fluke of
  Southeast Asia that causes shistosomiasis
• Humans are the definitive host snails are the
  intermediate host
• In humans its eggs ultimately penetrates and
  damages intestinal tissue and tissue of the
  bladder
• A source of constant inflammation and eventually
  leads to deterioration of liver, spleen and other
  organs

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• Class Cestoda
• General Morphology
• Nonciliated tegument composed of glycoprotein
• The anterior region is called a scolex often
  armed with suckers and hooks

• Extending from the neck is a series of
  proglottids contain the sex organs and eggs no
  digestive system
• Mature eggs released through an opening in the
  proglottid or leave the host when the proglottids
  are separated from the main body of the worm.

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• Beef Tapeworm, Taeniarhynchus saginatus
• Definitive host humans intermediate host cattle
• Eggs are shed with human feces infected persons
  defecate in a pasture and the eggs are ingested
  by cattle
• Eggs hatch giving rise to oncosphere larvae that
  bore into the intestinal wall and get into the
  circulatory system to be transported to muscle
• Here the larvae develop into the cysticercus
  stage (the bladder worm) with the inverted
  scolex
• If uncooked beef is consumed the cysticercus is
  freed and the scolex everts, forming the adult
• Symptoms include loss of weight, chronic
  indigestion, diarrhea

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