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Most of the species that have been named and studied belong to the animal kingdom - including well-known groups like mammals, birds, fishes, insects, and mollusks. Plants, the other relatively familiar kingdom, include fewer named species than beetles - a single order of animals! The number of named species does not necessarily indicate the number of species that actually exist, however. Recent work suggests that the greatest diversity of species occurs in the Kingdoms Protoctista (Protists) or Bacteria. The bacteria probably include two or more distinct kingdoms.

According to the Margulis and Schwartz (1998), animals are:

• heterotrophic (unable to synthesize their own food by photo- or chemo-synthesis)
• diploid (contain two sets of chromosomes)
• multicellular (comprised of more than one cell)
• include a blastula stage of early development (with the exception of sponges)
  

These traits are not necessarily unique to animals (e.g., plants and members of other kingdoms also are multicellular), but together they can be used to define the Animal Kingdom. A great variety of life forms exist in the animal kingdom, and this is the only group in which we find nervous systems and brains.

Animals range in size from microscopic zooplankton to whales, the largest animals ever to live on earth. Most species are found in the oceans, but of course animals are successful in land, water, and in the air. The number of phyla, classes, and other taxonomic categories changes as we learn more about relationships between species. Today, experts recognize about 37 different phyla of animals (Margulis and Schwartz 1998). Only 15 of these (indicated with an asterisk) are represented by species that occur in the Western Great Lakes Region. Links to other web resources are provided for some of the phyla. Many of these connect to the excellent site developed by the University of California-Berkely Museum of Paleontology.

• Phylum Placozoa (represented by a single marine species, Trichoplax adhaerens)
• Phylum Porifera* (sponges)

• Phylum Gnathostomulida (small animals of marine sediments)
• Phylum Rhombozoa (very small parasites of marine cephalopods)

• Radiata (radially symmetrical animals)
  • Phylum Cnidaria/Coelenterata* (hydra, jellyfish, corals)
  • Phylum Ctenophora (comb jellies)
    
    
• Acoelomates (bilaterally symmetrical animals without body cavity between gut and outer body muscles)
  • Phylum Platyhelminthes* (flatworms)
  • Phylum Orthonectida (small parasites of marine mollusks and polychaete worms)
  • Phylum Nemertina (ribbon worms; found mainly in marine sediments or benthos)
    
    
• Pseudocoelomates (bilaterally symmetrical animals with indistinct body cavity formed from embryonic cavity of blastula)
  • Phylum Nematoda* (roundworms)
  • Phylum Nematomorpha* (hair worms; found in freshwater, soil, and oceans)
  • Phylum Acanthocephala* (internal parasites of vertebrates and other animals)
  • Phylum Rotifera* (rotifers)
  • Phylum Kinorhyncha (small marine animals, most often found in sediments)
    
    
• Undetermined Group(s)(development of body cavity not yet described and classified)
  • Phylum Priapulida (benthic, marine worm-like animals)
  • Phylum Gastrotricha (unsegmented, wormlike animals of freshwater or marine benthos)
  • Phylum Loricifera (minute marine animals found attached to rocks and shells)
  • Phylum Entoprocta (small filter feeders found along ocean coasts; one freshwater genus)
  • Phylum Cycliophora (discovered in 1995 on marine lobster near Norway)
    
    
• Coelomates (bilaterally symmetrical animals with body cavity lined by cells derived from embryonic mesoderm)
  
  
  • Protostomes (mouth develops from first cavity formed during embryo development)
    
    
    Superphylum Arthropoda

    • Phylum Chelicerata* (spiders, scorpions, mites, horseshoe crabs)
    • Phylum Mandibulata* (insects, millipedes, centipedes)
    • Phylum Crustacea* (crayfish, shrimp, copepods, isopods, and relatives)
      
      
    Other Protostomes

    • Phylum Annelida* (segmented worms, including earthworms and polychaetes)
    • Phylum Sipuncula (peanut worms; found in shallow marine waters)
    • Phylum Echiura (spoon worms; burrowing marine worms)
    • Phylum Pogonophora (bearded tube worms; restricted to deep oceans)
    • Phylum Mollusca* (mussels, clams, snails, slugs, squids)
    • Phylum Tardigrada* (water bears; microscopic animals of oceans, freshwater, and damp terrestrial habitats like mosses)
    • Phylum Onycophora (velvet worms; found in forest regions of Southern Hemisphere)
      
      

    Lophophorates (development of mouth-anus orientation not established)

    • Phylum Bryozoa* (moss animals; colonial, found in oceans and freshwater)
    • Phylum Brachiopoda (lamp shells; found in cold water of deep or polar oceans)
    • Phylum Phoronida (horseshoe worms; found in shallow marine sediments)
      
      

    Deuterostomes (anus develops from first cavity formed during embryo development)

    • Phylum Chaetognatha (arrow worms; carnivorous marine zooplankton)
    • Phylum Hemichordata (acorn worms)
    • Phylum Echinodermata (starfish, sea cucumbers, sea urchins)
    • Phylum Urochordata (tunicates, sea squirts; larva resemble chordates)
    • Phylum Cephalochordata (amphioxus, lancelets; found in shallow oceans)
    • Phylum Craniata* (fishes, amphibians, reptiles, birds, mammals
      
      

Although spectacular fossils of wooly mammoths and dinosaurs have made news headlines, very few of the animals that have existed during the history of earth are preserved as fossils. In particular, soft-bodied animals like insects, worms, and protozoans are quickly decomposed and are unlikely to be fossilized except under extraordinary circumstances. Evidence from microfossils and marine sediments suggests that the first true multicellular animals (=metazoans) evolved on the ocean floor between 700 million and 800 million years ago. The oldest deposits of animal fossils were first found in the Ediacara Hills of South Australia, where animals resembling jellyfish, segmented worms (annelids) and arthropods occur in rocks dated at about 600 million years before present. The first "explosion" of animal taxa in the fossil record apparently took place during the Cambrian Period, about 500-570 million years ago (m.y.a.).

Several excellent web sites are devoted to information about animal diversity and the phyla listed above. Here we list some of the major sites. If you are aware of additional sites with high quality information about animals, please email the webmaster (address at the bottom of page) with your recoommendation.

Animal Diversity Web (University of Michigan Museum of Zoology)
Metazoan Diversity (Graham Davison)
University of California-Berkely Museum of Paleontology