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Ecdysozoa includes both pseudocoeolmate and coelomate animals.
Formation of a true coelom via schizocoely is currently considered to be derived with respect to formation of a pseudocoelom (persistent blastocoel).

Ecdysozoans undergo ecdysis, the shedding and replacement of the external cuticle as the animal grows.
The main structural component of the cuticle is chitin.

Current Ecdysozoan Phylogeny:

We will visit just a few of these hugely successful animals.

  • Priapulida - The maverick "Penis Worms"

  • Nematomorpha - The Horsehair Worms (parasitoids of arthropods)

  • Nematoda - The Roundworms
  • Onychophora - The Velvet Worms
  • Tardigrada - The Water Bears
  • Arthropoda - The Joint-legged Animals

    Nematoda (<--required link): The Roundworms

    The Nematode Bauplan:

    May be free-living or parasitic. Many are economically important agents of disease.

    Still, most nematodes are not only harmless, but beneficial members of soil communities.

    They are the most abundant animals on earth.

    Onychophora - The Velvet Worms

    These are tropical beauties that feed on small critters in the leaf litter.

    Tardigrada - The Water Bears

    The Tardigrade Bauplan

    Tardigrades have an amazing ability to survive physical conditions that would kill any other organism.

    ...don't seem to faze them!

    Investigators are studying the tardigrades' amazing abilities to undergo cryptobiosis.

    Discovering their secret could help us learn how to protect mammalian cells from extreme conditions.
    Could discovering the tardigrades' secrets help us prevent damage to mammalian cells under such extreme conditions?
    Maybe we could travel through space and colonize other planets (so we can wreck them, too).

    Arthropoda - The Joint-footed Animals

    Arthropods inhabit every conceivable environment, from the open ocean, to the bodies of other animals, plants, and fungi (even your eyelash follicles, home to Sarcoptes mites that feed on your dead skin).

    Their evolutionary relationships have undergone a great deal of revision in the past decades.

    The Arthropod Bauplan

    What are the relative advantages of each of these types of development?

    Arthropod Diversity

    Trilobites - The Trilobites (all extinct)


    The Chelicerates are united by both molecular and morphological similarity.
    All have mouthparts known as chelicerae, which may be highly modified for different feeding natural histories.

    Chelicerate Diversity

  • Xiphosura - The Horseshoe Crabs

  • Arachnida - Spiders, scorpions, mites & ticks

  • Pycnogonida - The Sea Spiders

    Myriapoda - Centipedes Millipedes, and their cousins

    Hexapoda - The Insects and their cousins Crustacea

    This diverse group is primarily aquatic, though there are a few terresterial species.
    Some of the more familiar groups: