Chapter 13 - Introduction to Phylum Platyhelminthes

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Chapter 13 - Introduction to Phylum
Platyhelminthes
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• General Characteristics
• Bilaterally symmetrical dorsoventrally
  flattened
• Acoelomates most of the body is comprised of
  parenchyma, a collection of fibers and cell types
  (sensory, food storage, etc.) internal organs
  are embedded in the parenchyma muscle fibers are
  also found running through the parenchyma
• Unable to synthesize fatty acids and sterols de
  novo

• Turbellaria and free-living stages (certain
  larval stages) of Class Cestoidea and Trematoda
  have ciliated epithelium

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• General Characteristics cont.
• The nervous system does have a centralized
  (ganglionated) area and a peripheral system
  (arranged as a network or as a ladderlike system
  with longitudinal nerve cords and transverse
  commissures)
• The digestive system is typically a blind sac
  and most flatworms have a mouth near the anterior
  end followed by a muscular pumping pharynx

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• General Characteristics cont.
• Anus absent wastes are eliminated through the
  mouth or via nephridiopores
• In some the gut is a simple sac, while in others
  it is finely branched structure some groups
  (cestodes) the gut is lacking

• Osmoregulatory system is comprised of flame
  cells or protonephridia water and waste exit the
  body through pores called nephridiopores

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• General Characteristics cont.
• Most species are monoecious, with cross
  fertilization
• In some turbellarians and cestodes groups sperm
  transfer is by hypodermic impregnation

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Platyhelminths Systematics Traditional
Classification Scheme 1. Class Turbellaria
Most free-living worms from terrestrial, FW and
marine environments 2. Class Monogenea All
parasitic, mainly ectoparasites of fishes (e.g.
found on the skin and gills) 3. Class Trematoda
All parasitic, mainly in the digestive tract of
vertebrates a. Subclass Digenea - at least a 2
host life cycle first host is a mollusc b.
Subclass Aspidogastrea - most with only one host
(mollusc) c. Didymozoidea - tissue
dwelling parasites of fish 4. Class Cestoidea
All parasitic in all classes of vertebrates
except agnathans most have an intermediate host
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Characteristics of Revised Classification Scheme

• The subphylum Catenulida is a sister group of
  the true Platyhelminthes
• The main structural feature separating the
  subphylum Catenulida from the subphylum
  Euplatyhelminthes is the presence of a frontal
  organ in all the latter Euplatyhelminthes also
  possess dense dermal ciliature compared to
  catenulids

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• Revised Classification Scheme cont.
• Among the S.P. Euplatyhelminthes, the acoelomate
  forms (Superclass Acoelomorpha) lack a
  protonephridia and generally do not have a gut
• The Superclasses Rhabditophora have some kind of
  protonephtridia

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Taxonomic groups that will be emphasized
Superclass Rhabditophora Class Trematoda
Subclass Aspidobothrea Subclass
Digenea Class Monogenea Class Cestoidea
Subclass Cestodaria Subclass Eucestoda