chapter 13 introduction to phylum platyhelminthes n.
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Chapter 13 - Introduction to Phylum Platyhelminthes. General Characteristics Bilaterally symmetrical; dorsoventrally flattened

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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

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

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

General Characteristics cont.

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

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


c. Didymozoidea - tissue dwelling parasites of fish

4. Class Cestoidea: All parasitic in all classes of vertebrates except agnathans; most have an intermediate host


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

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

Taxonomic groups that will be emphasized

Superclass Rhabditophora

Class Trematoda

Subclass Aspidobothrea

Subclass Digenea

Class Monogenea

Class Cestoidea

Subclass Cestodaria

Subclass Eucestoda