Phylum Platyhelminthes

Phylum Platyhelminthes

Phylum Platyhelminthes
Class Turbellaria
– Class Cestoda
– Class Trematoda

+ Phylum Platyhelminthes features 

  • Species in this categories are triploblastic (have three layers of skin), acoelomate, bilaterally symmetric.
  • They have protostome (lophotrochozoan)
  • They have a central nervous system and go through cephalization.
  • They have a digestive system with a single opening (no anus) due to being bilateral symmetric.
  • They have an excretory system called protonephridia that filters the blood; it works like our kidney.
  • Most are simultaneous hermaphrodites (monoecious)
  • It includes both free-living and parasite species

+ Flatworms are apart of the phylum Platyhelminthes, they are flat because it gives them an advantage for respiration and circulation; so gas exchange occurs by diffusion across the body surface. They do not have blood like the way we have blood.

+ There are four main classes to this phylum: Monogenea, Turbellaria, Cestoda, and Trematoda.

+ Class monogenean, Cestoda, and Trematoda are mostly parasites.

+ Class turbellaria have ciliated epidermis and monogenean, Cestoda, Trematoda are syncytial tegument and non-ciliated.

Class Turbellarian 

+ Needs watery environment to move around in and varies in their size. They have a pharynx; a tube that extends from the gastrovascular cavity (how they eat), gastrovascular cavity, eyespots, ganglia and ventral nerve cords.


+ The protonephridia system is a system of tubules (excretory canal) that filter body fluids and waste elimination and out the excretory pore (duct).

+ Their nervous system includes a cerebral ganglion, pharynx, a ventrolateral, lateral, ventral and dorsal cord with a ring commissures.

+ They have sensory receptors (e.g; rheoreceptoes & chemoreceptors) and eyespots. 

+ Their epidermis and body wall have basal bodies of cilia, ducts of glands, rhabdoids, nuclei and a membrane with circular muscles. Below the membrane, they have diagonal muscles, dorsoventral muscles, and longitudinal muscles.

+ They have a genital pore; their reproductive system includes both testes and ovaries and they reproduce asexually.

+ Adaptations of a Parasitic Lifestyle (Monogenea, Cestoda, Trematoda)

  • Evolution of holdfast organs
  • Reduction of the nervous system, loss of cephalization, loss of sensory structure because they are in areas/conditions that don’t require them to carry these traits.
  • Loss of locomotor organs
  • Reduction of digestive system; absorb nutrients through body wall
  • Greatly increased reproductive capacity, elaboration of reproductive organs; hermaphroditism, parthenogenesis (Growing into an adult, e.g; like ants)
  • Primary, secondary and tertiary hosts (a lot of them will have very complicated life cycle)
  • Mechanisms to avoid host immune system





By Alex Popovkin, Bahia, Brazil licensed under CC BY 2.0


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