ASCERTAINMENT BIAS An error that is introduced with a biased sampling scheme.

BILATERIANS Multicellular animals that have a real body cavity (coelom) and a primary bilateral symmetry. They include all multicellular organisms except for the sponges, cnidarians and ctenophorans.

CHOANOFLAGELLATES A group of protists that contain one flagellum at some stage of their life history.

CLADE A taxon or other group of organisms that share a closer common ancestor with one another than with members of any other clade.

CNIDARIANS A simple and ancient phylum of multicellular animals, such as jellyfish or corals, found mainly in marine environments.

CYCLOSTOMES A group of ancestral jawless fishes, including the lampreys.

DEUTEROSTOMES A bilaterian clade that is characterized by the formation of distinct mouth and anal openings.

DIPLOBLASTS A group of ancestral animals, such as the cnidarians and the porifera, that do not develop mesoderm.

DOSAGE COMPENSATION A mechanism that regulates the expression of sex-linked genes that differ in dose between females and males.

ECDYSOZOA A bilaterian clade that is characterized by external cuticles that are shed during stages of development. It includes the insects and nematodes.

EUMETAZOANS An animal subkingdom that includes the cnidarians, the ctenophorans and the bilaterians.

GNATHOSTOMES The group of higher fishes, all of which are characterized by the presence of jaws.

HETEROTOPY The displacement of the development of an organ in space.

KM The substrate concentration at which the reaction rate of an enzyme is half maximal, also known as the Michaelis–Menten constant.

LOPHOTROCHOZOA A bilaterian clade that is characterized by a lophophore (a specific morphological structure) or a trochophore larval stage. Well-known members include the molluscs and the annelids.

NEOMORPHIC A qualitatively new feature of a phenotype that is produced by a mutant allele.

PARALOGUES Homologous genes that have originated by gene duplication.

PARAZOANS An animal subkingdom that includes the porifera and the placozoa, the latter of which contains only one species (Trichoplax).

PORIFERA A phylum of multicellular animals with only two cell layers, the ectoderm and the endoderm, that are separated by an acellular mesogloea.

PROTISTS Unicellular heterotrophic eukaryotes.

PROTOSTOMES Animals whose development is characterized by the formation of a single opening. The protostomial phyla are subdivided into the ecdysozoans and the lophotrochozoans.

PSEUDOCOELOMATES Animals, such as the nematode, that do not have a body cavity that is fully lined with mesodermal cells.