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The Ecdysozoa are a group of phyla that include the two largest
phyla of all, Arthropoda and Nematoda, along with several smaller phyla.
All Ecdysozoa have a tough, chemically resistant outer cuticle that
must be shed periodically during growth; the shedding or molting process
is under the control of steroid hormones (ecdysones).
Most ecdysozoan phyla have a pseudocoel, but the arthropods have a true coelom
of the schizocoel type. The Nematoda (roundworms) are very small
worms with tapering ends.
The Arthropoda, the largest and most successful phylum of organisms,
are characterized by a tough exoskeleton and jointed legs. Major groups of
arthropods include the extinct trilobites, the largely aquatic Crustacea,
the chelicerate groups (spiders, mites, scorpions, horseshoe crabs, etc.),
the centipedes, the millipedes, and the insects. The insects alone make up
about 2/3 of all the living species on Earth.
Phylum Nematoda (roundworms) are very small worms with tapering ends, living very abundantly in all environments. Some are free-living, but many are parasitic on plants or animals. Caenorhabditis elegans is a free-living nematode often used in biological research. Filaria is a genus of parasitic roundworms that causes elephantiasis and other human diseases. Trichina and Trichinella are parasitic roundworms that can enter human bodies through insufficiently cooked beef or pork. Phylum Gordiacea or Nematomorpha: Very long, thin "horsehair worms." Phylum Cephalorhyncha includes the groups Priapulida, Kinorhyncha, and Loricifera. They are all small and live in marine environments. All have a proboscis, covered with spines, that can either be withdrawn or turned inside out and everted. Phylum Pentastomida: Endoparasites inside vertebrates, with 2 pairs of short, degenerate legs armed with claws. Phylum Tardigrada: Tiny "water bears," with 8 short legs ending in claws. Phylum Onychophora: An ancient group (Cambrian to Recent), closely related to arthropod ancestors. Segmented, wormlike body. Numerous short feet (1 per segment), ending in claws. Feet around mouth function in seizing and tearing food. Phylum Arthropoda: Animals with a tough exoskeleton, often strengthened by calcium salts, and jointed legs with movable joints between rigid segments. Metamerism (segmental organization), but segments differ very much regionally. Mouthparts often derived from legs. Open circulatory system. Several anterior segments commonly coalesced into a head. Nervous system reminiscent of annelids, with ventral nerve cord, esophageal ring, and dorsal brain. Included groups:
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