Organismal Biology #24
ARTHROPODS AND OTHER ECDYSOZOA

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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). Many 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 All have a proboscis, covered with spines, that can either be withdrawn or turned inside out and everted.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.

Ecdysozoa


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:
  • Trilobites: Extinct, marine arthropods with numerous similar biramous (two-branched) appendages, each with a leg-like part and a feathery gill. Body divided into cephalon (head), thorax, and pygidium. May be ancestral to other arthropods (but experts disagree on this). Flat-bottomed shape shows that most trilobites were bottom-dwellers.
  • Crustacea: A largely marine group of arthropods, breathing by gills. Always two pairs of antennae. First post-oral segment has a pair of mandibles. Appendages biramous (two-branched), as in trilobites. Includes lobsters, crabs, shrimp, barnacles, and many other species.
  • Chelicerate groups: Originally marine arthropods, but one large group successfully invaded land environments. No antennae are present. First 2 pairs of appendages include a pair of chelicerae (piercing structures that may be used to inject venom) and a pair of pedipalps which may hold prey while the chelicerae pierce them. Many chelicerates are predators. Body usually divided into cephalothorax and abdomen (except in mites). Includes Pycnogonida (sea spiders), Xiphosura (horseshoe crabs), and Arachnida (scorpions, spiders, mites, etc.).
  • Uniramous "myriapods": Arthropods with elongate, worm-like bodies and many pairs of legs. Includes centipedes (Chilopoda), millipedes (Diplopoda), and two other groups (Pauropoda, Symphyla) allied to insects. All myriapods have similarities that they share with insects: a single pair of antennae; a pair of mandibles; uniramous appendages (having only a single branch); respiration using a tracheal system of air-tubes. Habitats usually terrestrial, often hiding beneath rocks and rotting logs.
  • Insects: The largest and most successful group of arthropods, including about 75% of all species in the entire animal kingdom. Mandibles, uniramous appendages, and tracheal systems as in myriapods. Division of the body into three portions: head (containing antennae, mouth, and compound eyes), thorax (typically with three pairs of walking legs and two pairs of wings), and abdomen or 7 or more segments (containing reproductive structures).
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