Phylum Platyhelminthes Overview

Phylum Platyhelminthes Summary

• Platyhelminthes are a phylum of flatworms
• Members of this phylum are primarily free-living, or parasitic
• The most prominent feature of platyhelminths is their flattened shape
• They lack specialized respiratory and circulatory systems
• Gas exchange occurs via simple diffusion across the body surface
• Metabolic wastes primarily diffuse across the body surface
• Specialized organs called protonephridia are responsible for removing metabolic wastes like ammonia, urea, and amino acids
• Osmoregulation is achieved through protonephridia
• Most platyhelminth species are simultaneous hermaphrodites

Platyhelminthes Classes

• Class Turbellaria:
  • About 6% of all flatworm species are turbellarians
  • Predominantly free-living
  • Primarily inhabit marine, freshwater, and terrestrial environments (humid)
  • Mostly less than 1cm in length
  • Most aquatic turbellarians are benthic, living on the ocean, lake, pond or river bottom
  • Have a ciliated outer body surface, especially on the ventral side
  • Movement is often via mucus secretion with cilia movement within the mucus
• Class Cestoda:
  • Members of this class are primarily endoparasitic
  • Commonly known as tapeworms
  • Composed of proglottids, segments that develop from the anterior end
  • Characterized by a scolex, an anterior hooked attachment organ
  • The vast majority of species are simultaneous hermaphrodites
  • Usually inhabit various regions of the host's digestive tract
  • Contain numerous nuclei within the tegument, separated by cell membranes(syncytial)
  • The outer surface of the tegument is out folded into cytoplasmic projections, greatly increasing exposed surface area for nutrient absorption
• Class Monogenea:
  • Monogenetic flatworms are typically ectoparasites, residing on the skin or gills of fishes
  • Show a high degree of host specificity and reside in specific host sites
• Class Trematoda:
  • Trematodes have a mouth opening and a blind-ended digestive tract, typically bilobed
  • Tegument is unciliated and syncytial
  • Body is not segmented
  • Parasites that ingest host tissue and blood (e.g. Schistosomiasis blood flukes)

Defining Characteristics of Different Classes

• Class Cestoda:
  • Small anterior hooked attachment organ (scolex)
  • Body divided into segments (proglottids) arising from anterior end
• Class Trematoda:
  • Posterior attachment organ (haptor)
  • Larval stage (oncomiracidium) bearing 3 bands of cilia.
  • Typically bearing 1 or 2 pairs of eyes.
• Class Monogenea:
  • Mostly parasitic on the skin or gills of fish.