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The Ecdysozoa

This clade includes the familiar and unfamiliar phyla

We'll visit just a few of these.

PHYLUM NEMATODA: The Roundworms


Most of the nematodes we notice are those that cause disease...

But far more nematodes are not only harmless, but helpful members of soil ecosystems. They are the most numerous animals on earth. It has been said that if one were to remove all the tissues of the biosphere except that of nematodes, one could still see the general outline of every living thing just from the biomass of nematodes.

PHYLUM ROTIFERA: The Wheel Animalcules

PHYLUM TARDIGRADA - The Water Bears

Like the other Ecdysozoans, tardigrades undergo ecdysis. Like the next group we'll see, they have somewhat jointed appendages. But of greatest interest to scientists is the tardigrades' amazing ability to survive physical conditions that would kill any other organism.

...don't seem to faze them!

Scientists are studying tardigrades and their amazing abilities to undergo "suspended animation" to find out how to protect mammalian cells from extreme conditions. The hope is that learning these secrets might help us travel through space and colonize other planets--or any other fantasy-like idea you could name.

PHYLUM ARTHROPODA - The Joint-footed Animals

  • Arthropods inhabit every conceivable environment, from the open ocean, to the bodies of other animals (including your eyelash follicles).

    All Arthropods are characterized by

    • all characters of protostome coelomates
    • metamerism, both internal and external
    • fusion of body segments (TAGMOSIS)
      • head
      • thorax
      • abdomen
    • external skeleton composed of chitin (and sometimes fortified with calcium carbonate, especially in marine forms)
    • one pair of appendages per each true body segment
    • most forms have at least one pair of lateral, COMPOUND (faceted) EYES composed of individual photoreceptor units called OMMATIDIA (singular = ommatidium)
    • coelom reduced to a gonocoel
    • main body cavity is the HEMOCOEL, part of the OPEN CIRCULATORY SYSTEM
    • Nervous system ventral and much like the annelid system
    • Muscles striated and arranged in segmental bands linking the plates of the exoskeleton; highly efficient locomotion
    • Growth accompanied by ECDYSIS (molting)
    • Development and metamorphosis (changing from juvenile to adult form) may be
      • complete
          Arthropods (and other animals) undergoing complete development have larvae that looks distinctly different from the adult, and which must undergo a rather drastic metamorphosis to change into the adult form. EXAMPLE: butterfly

      • simple

          Arthropods (and other animals) that undergo direct development do not have a free-living larval form. Rather, the parent animals care for the babies, usually by brooding or encapsulating them (in eggs), and the young have the same form as the adult, but smaller. A small version of the adult is sometimes called a NYMPH.
          EXAMPLES: milkweed bug, silverfish

      • mixed

          Arthropods (and other animals) that undergo mixed development brood or encapsulate the zygotes in eggs, which hatch as larvae with a form somewhat--but not drastically-- different from that of the adult. Subsequent metamorphic changes generate the fully adult form.
          EXAMPLE: dragonfly (egg --> nymph --> adult)
          (Note: an aquatic nymph larva--like that of a dragonfly or damselfly--is called a NAIAD)

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