Table 1.

List of major animals discussed in this review.

Protozoa	Ciliates (e.g., Paramecium, Tetrahymena, suctorians) Choanoflagellates
Basal Metazoans	Porifera (sponges) Placozoa Ctenophora (comb jellies) Cnidaria (hydras, jellyfish, sea anemones, colonial hydroids, corals) Myxozoa
Bilateria	Acoelomorpha (acoel flatworms) Platyhelminthes (planaria and other free-living flatworms, flukes, tapeworms) Other groups included for comparison: Nematoda (Caenorhabditis elegans) Annelida (polychaete worms) Mollusca (snails, clams, oysters) Arthropoda (Crustacea, Drosophila) Chordata (sea squirts, vertebrates such as fish, frogs, mice, humans)