Phyla Nemertea to Arthropoda

Phylum Nemertea (ribbon worms)

  • Small group (about 900 species) of free-living animals.
  • Mostly marine, some in fresh water or damp soil.
  • Long, narrow bodies (5 cm to 2 m).
  • Possess a long proboscis for catching prey using mucus.
  • Acoelomate except around the proboscis.
  • Have a circulatory system, mouth, and anus.

Phylum Nematoda (nematodes or roundworms)

  • Triploblastic with bilateral symmetry.
  • Elongated, round worms with pointed ends; have an alimentary canal.
  • Sexes are separate.
  • Some are free-living, many are parasites.
  • Pseudocoelomates without a blood system.
  • Anterior end shows cephalization.
  • Play important roles in:
    • Pest control
    • Soil fertility (Nitrogen cycle)
    • Decomposition
    • Bioindicators
  • Some are pathogenic to humans and animals (e.g., Ascaris, Wuchereria bancroftii).

Phylum Rotifera (Rotifers) – “Wheel animals”

  • Aquatic, microscopic animals mostly in freshwater.
  • Have a crown of cilia on their anterior end that looks like a spinning wheel.
  • Pseudocoelomates with a complete digestive tract.
  • Have a nervous system with a "brain" and sense organs.

Phylum Mollusca (Mollusks)

  • Soft body, usually covered by a dorsal shell (calcium carbonate).
  • Have a muscular foot for locomotion.
  • Visceral mass located above the foot.
  • Have a mantle.
  • Mostly marine.
  • Major classes:
    • Polyplacophora (Chitons)
    • Gastropoda (Snails, slugs)
    • Bivalvia (Clams, oysters, mussels)
    • Cephalopoda (Squids, octopods)

Phylum Annelida (annelids or segmented worms)

  • Tripoblastic, coelomate with bilateral symmetry.
  • Metameric segmentation.
  • Have a prostomium and cuticle.
  • Chaetae present (except in leeches).

Phylum Arthropoda (arthropods)

  • Most species-rich phylum.
  • Exploit all habitats.
  • Underwent adaptive radiation to fill various ecological niches.
  • Segmented like annelids, but with a fixed number of segments.
  • Share a similar nervous system plan with annelids (ventral nerve cord).
  • Exoskeleton of chitin, jointed appendages.
  • Reduced coelom, main body cavity is a haemocoel.