Platyhelminthes: Flatworm Notes

Platyhelminthes: Flatworms

  • Flatworms (Platyhelminthes) are being discussed, considering the implications of body size and shape for life's activities.
    • Surface area to volume ratio is a key consideration.
    • Locomotion and symmetry are important factors.
    • Cephalization, the concentration of sensory organs and nervous control at the anterior end, is also relevant.

Overview of Platyhelminthes

  • Comparison with Other Groups: Platyhelminthes are compared with Porifera, Cnidaria, Chelicerata, and Insecta (Class).
  • Life Processes: Key aspects of life are examined:
    • Obtaining food
    • Reproduction
    • Gas exchange
    • Excretion / osmoregulation
    • Locomotion / support
  • Body Plan: The body plan of flatworms is discussed, including organs and tissues, and totipotent cell types.
  • Phylogeny: The phylogeny of bilateral animals is referenced (Sodergren et al., 2006, Science 314: 941-952).
  • Position in Phylogeny: Flatworms are sometimes placed outside the Lophotrochozoa in the Rouphozoa.

Phylogeny and Classes of Platyhelminthes

  • Phylogenetic Placement: The placement of Platyhelminthes (Dugesia) in the tree of life is discussed, referencing Srivastava et al. (2010) Nature doi:10.1038/nature09201, and Dunn et al. (2008) Nature Letters doi:10.1038/nature06614.
  • Ecological Niches: Flatworms occupy various niches:
    • Fish and amphibian ectoparasites
    • Free-living flatworms
    • Parasitic flukes
    • Tapeworms
  • Classes of Platyhelminthes:
    • Class: Turbellaria: Predominantly aquatic (marine or fresh water) or interstitial fauna among wet rocks/sand, covered by ciliated epidermis, pharynx. Contains 4500 species and may be polyphyletic.
    • Class: Monogenea: Monogenetic flukes, covered by tegument, oral sucker reduced or absent, pro- and opisthaptors present, ectoparasitic, 1100 species.
    • Class: Trematoda: Digenetic flukes, body covered by tegument, one/more oral suckers, haptors absent, two or three hosts, endoparasitic, 11000 species.
    • Class: Cestoda: Tapeworms, covered by tegument, scolex and strobila of proglottids (“segments”), digestive tract absent, two/three hosts, endoparasitic, 3400 species.

Examples of Turbellaria and Monogenea

  • Class Turbellaria: Examples include:
    • Pseudoceros ferrungineus
    • Bipalium simrothi
    • Dugesia tigrina
    • Eurylepta californica
    • Reference to Wong and Gorb (2013) J Exp Biol 216:3008-3014
  • Class Monogenea

Examples of Trematoda

  • Class Trematoda: Examples include:
    • Fasciola gigantica adult from bovine liver
    • Chlonorcis sinensis adult from human bile duct
    • Schistosoma mansoni cercarium
    • Cercarial dermatitis from Austrobilharzia
    • The Digenean - Dicrocoelium dendriticum

Schistosomiasis

  • Schistosomiasis (bilharzia): An acute and chronic neglected tropical disease.
    • Infection Statistics: Widespread in poor communities, affecting 221 million people worldwide, with 9 out of 10 infected people living in Africa.
    • Transmission: Spread through open defecation and urination, with cercariae penetrating the skin from infested water.
    • Disease Forms: Intestinal and urogenital.
    • Female Genital Schistosomiasis: Identified as a major risk factor for HIV infection.
    • Economic Consequences: Devastating health and economic impacts.
    • Affected Populations: Women, children, manual workers, and fishermen.
  • Infection Cycle:
    • People become infected when larval forms of the parasite (released by freshwater snails) penetrate the skin during contact with infested water.
    • Cycle includes cercariae entering the blood, affecting the intestinal and urogenital tracts.
    • Intermediate host (snail) releases miracidia.
    • Eggs are present in stool and urine.
  • Prevention and Control:
    • WHO recommends praziquantel for treatment.
    • Preventive chemotherapy, snail control, environmental management, health education, social mobilization, sanitation, and access to safe water are crucial.

Cestoda

  • Class Cestoda: Examples include:
    • Taenia taeniaeformis
    • Taenia pisiformes
  • Anatomy: Scolex, neck, and strobila, where the strobila is an elongated body made up of segments called proglottids.
  • Proglottids: Segments shed into the environment.
  • Reproduction: Cestodes are hermaphroditic and lack a digestive system, living for years in the final host. Reproduction occurs in the Class Cestoda.

Animal Architecture (Bauplan)

  • Definition: Animal body plan or Bauplan is described as an assemblage of morphological features shared among many members of a phylum-level group (Valentine, 2004, On the Origin of Phyla).
  • Number of Phyla: There are 34 animal phyla (Ruggiero et al., 2016, doi:10.1371/journal.pone.0119248).
  • Evolutionary Context: These 34 are nearly all remaining descendants of the approximately 100 phyla present in the Cambrian explosion.
  • Example: Planarian three-layered cross-section, diagrams from “Animals without Backbones” by Buchsbaum et al.

Planarian Digestive System

  • Feeding: Food may be sucked in through the mouth, or the pharynx everted (cilia and digestive cells on surface).
  • Waste Excretion: Fecal matter is also excreted through the mouth.
  • Liquid Waste Excretion: Liquid waste is excreted through the cell walls/pores.
  • Gut Complexity: The gut varies in complexity from a simple single tube to a complex branched intestine; parasitic forms often have a simple two-branched gut.
  • Gut Reduction: Some phylogenetic studies suggest that the gut has been diminished or lost in parasitic species; cestodes have no gut at all and absorb nutrients directly through the tegument.

Planarian Nervous System

  • Nervous System Components:
    • Bipolar nerve cells
    • Eyespot
    • Auricle (mechanosensory and chemosensory)
    • Interneuron
    • Ventral nerve cord
    • Ganglia
    • Commissural fibers
  • Sensory Structures:
    • Eyespots for light sensing.
    • Auricles for mechanosensory and chemosensory functions.
  • Other Features:
    • Pharynx
    • Pigment cells
    • Photoreceptors

Planarian Circulation, Gas Exchange, Excretion, and Osmoregulation

  • Circulation and Gas Exchange: No specialized circulatory structures; diffusion limited.
  • Excretion and Osmoregulation: Osmoregulation and excretion by protonephridia.
    • Flame Cells: Beating cilia in flame cells cause a current, fluid is filtered as it passes through the cell and tubules.
    • Function: Removes excess water but retains essential ions.
    • Nephropore: Tube cell opens through a nephropore to the external environment.
    • Marine Species: Absent or reduced in marine species as there is no need to get rid of water.

Planarian Reproductive System

  • Reproductive Organs: Includes yolk gland, copulatory sac, oviduct, ovary, testis, sperm duct, penis, and genital chamber.
  • Reproduction in Turbellaria: Asexual reproduction via fission and budding.

Planarian Summary and Ecology

  • Cross-Section Summary: Comprehensive overview of internal structures.
  • Ecology:
    • Free-living flatworms are an important part of marine, freshwater, and terrestrial ecosystems.
    • In marine systems, they can make up as much as 25% of the meiofauna.
    • In some areas, their biomass is equal to that of nematodes.
  • Economic Importance:
    • The terrestrial New Zealand flatworm is an invasive predator in Europe, threatening earthworm populations, soil integrity, and food security.
    • Planarians are an important model in stem cell science.
    • Parasitic species have medical, veterinary, and wildlife significance.
    • The number of people treated for schistosomiasis rose from 12.4 million in 2006 to 33.5 million in 2010 (according to WHO).

Summary – Platyhelminthes

  • Phylum Characteristics:
    • Bilateral symmetry
    • Triploblasty (3 germ layers) – but are acoelomate
    • Cephalization
    • Basic digestive and excretory system (no digestive system in cestodes)
    • Brain and differentiated nervous system
    • Sexual and asexual reproduction
  • Class Turbellaria:
    • Mostly free-living marine flatworms (some freshwater and a few land), making up a substantial portion of the marine ecosystem.
  • Other Classes:
    • The other three Classes are parasitic, many species of health and economic importance.
    • Several examples of behavioral manipulators among the parasitic classes.
  • Planarians:
    • Of the freshwater turbellaria, planarians are the best known.
    • This group is very important for our understanding of stem cells and regeneration.