The Ecdysozoa
This clade includes the familiar and unfamiliar phyla
- Phylum Nematoda - The Roundworms
- Phylum Rotifera - The Wheel Animalcules
- Phylum Cephalorhyncha - The Cephalorhynchs
- Phylum Nematomorpha - The Horsehair Worms
- Phylum Onychophora - The Velvet Worms
- Phylum Tardigrada - The Water Bears
- Phylum Arthropoda - The Joint-legged Animals
We'll visit just a few of these.
PHYLUM NEMATODA: The Roundworms
- pseudocoelomate (body cavity is a persistent blastocoel)
- has only longitudinal muscles in body wall, resulting in
characteristic sinusoidal body motion.
- hugely abundant
- most are free-living, but many are parasitic
- unsegmented, the pseudocoelom is a continuous, fluid-filled
cavity
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
- complex animals with all pseudocoelomate characters
- no bigger than a large protist!
- characterized by a cephalic, ciliated corona used for feeding and
locomotion
- mouth contains complex grinding apparatus called a MASTAX
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.
- extreme heat and cold
- nearly 100% desiccation
- vaccuum conditions (zero oxygen!)
- desiccation for hundreds of years
...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)
- 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?
