Platyhelminthes and Annelids

Bilateria

  • A large clade that contains almost all triploblastic animals
  • Further subdivided into two clades:
    • Protostomes
    • Deuterostomes

Protostomes include:

  • Platyhelminthes (flatworms)
  • Annelida (segmented worms)
  • Mollusca
  • Nematoda
  • Arthropoda

Deuterostomes include:

  • Echinodermata
  • Chordata

Platyhelminthes (Flatworms)

  • Characteristics:

    • Consists of free-living and parasitic species
    • Most free-living are aquatic; terrestrial flatworms are limited to moist areas
    • Dorsoventrally flattened bodies
    • Acoelomate (no body cavity)
    • Many parasitic flatworms have more than one host organism:
    • First host: usually an invertebrate
    • Final host: usually a vertebrate

  • Debate on Phylum Validity:

    • No clear defining feature found across all members of the phylum

Physiology of Flatworms

  • Free-living flatworms are either predators or scavengers, while parasitic flatworms feed on host tissues

  • Most have an incomplete digestive system; a single opening is used for both eating and expelling waste

  • Digestion is extracellular

  • Tapeworms lack a digestive system entirely

  • Excretory system:

    • Simple, uses flagellated flame cells in tubules to direct waste

  • Lack respiratory and circulatory systems:

    • Gas and nutrient exchange via diffusion
    • Necessitates a flat body plan

  • Reproductive Characteristics:

    • Most species are monoecious/hermaphroditic with internal fertilization
    • Some can reproduce asexually via regeneration

  • Nervous System:

    • Simple organization with longitudinal nerve cords and transverse nerves (ladder pattern)
    • Concentration of nerve cells at the anterior end with sensory cells present

Flatworm Diversity

  • Four Classes:
    1. Turbellaria
    • Mostly free-living
    • Inhabit marine, freshwater, and moist terrestrial habitats
    • Movement via cilia and musculature
    1. Monogenea (monogenetic flukes)
    • Parasites mainly on fish
    • Attach to hosts with hooks
    • Single host in life cycle
    1. Trematoda (flukes)
    • Internal parasites with primary and secondary hosts
    • Primary host: usually a mollusk
    • Secondary hosts: vertebrates
    • Causative Diseases:
      • Schistosomiasis caused by the blood fluke Schistosoma
      • Common in tropical areas with poor water quality, prevalent in the Philippines
    1. Cestoda (tapeworms)
    • Internal parasites residing in host digestive tracts
    • Lack a digestive system; attach via suckers on the scolex
    • Reproduction through proglottids (segments) that detach and exit with feces
    • Human infections through poorly cooked pork, beef, and fish consumption

Annelida (Segmented Worms)

  • Characteristics:

    • Exhibit true segmentation (metamerism);
    • Repeated features in both external and internal morphology
    • Coelomate

  • Habitat:

    • Inhabit marine, freshwater, and terrestrial environments (especially soil)

Physiology of Annelids

  • Each segment equipped with chaetae/setae for movement

  • Musculature includes two layers:

    1. Circular
    2. Longitudinal

  • Complete digestive system with specialized organs

  • Metanephridia facilitates excretion

  • Gas exchange happens cutaneously

  • Circulatory System:

    • Closed circulatory system with aortic arches acting as hearts
    • Well-developed nervous system with:
    • Two ventral nerve cords
    • Nerve ring around the pharynx

  • Reproductive Characteristics:

    • Monoecious: have permanent gonads
    • Dioecious: have temporary gonads

Annelid Diversity

  • Two Major Classes:

    1. Polychaeta (bristle worms)
    • Mostly marine with multiple chaetae per segment
    • Paired, fleshy appendages (parapodia) present
    • Lacks clitellum
    1. Clitellata
    • Fewer/reduced chaetae or none; presence of clitellum which secretes mucus for reproduction
    • Subclasses:
      • Oligochaeta: Earthworms with reduced setae
      • Hirudinea: Leeches lacking setae, with suckers for predation/ parasitism

  • Importance of Earthworms:

    • Mix topsoil and subsoil, adding nitrogenous products during ingestion

  • Historical and Current Use of Leeches:

    • Used in bloodletting and modern medical applications to alleviate congestion during tissue healing.

References

  • Clark MA, Douglas M, Choi J. 2024. Biology 2e. OpenStax.
  • Fowler S, Roush R, Wise J. Concepts of Biology. OpenStax.
  • Hickman C, Keen S, Eisenhour D, Larson A, l’Anson H. Integrated Principles of Zoology. NY: McGraw Hill.