Blastomere (Gr. blastos, germ, + meros, part) An early cleavage cell.

Blastule (Gr. blastos, germ, + L. ula, dim.) Early embryological stage of many animals; consists of a hollow mass of cells.

Clade (Gr. klados, branch, sprout) A taxon or other group consisting of an ancestral species and all of its descendants, forming a distinct branch on a cladogram or phylogenetic tree.

Cladogram (Gr. klados, branch, + gramma, letter) showing the pattern of sharing of evolutionarily derived characters among species or higher taxa.

Class One of the levels in the scientific classification of the organisms with taxonomic rank between phylum and order.

Endoparasite A parasite that resides inside the body of its host organism; contrasts with ectoparasite.

Family Group in the zoological classification of the organisms with taxonomic rank between order and genus. A family consists of various related genera.

Genus (Lat. genus, race; pl. genera) A group of related species with taxonomic rank between family and species.

Monophyly (Gr. monos, single, + phylon, tribe) The condition that a taxon or other group of organisms contains the most recent common ancestor of the group and all of its descendants; contrasts with polyphyly and paraphyly.

Paraphyly (Gr. para, beside, + phylon, tribe) The condition that a taxon or other group of organisms contains the most recent common ancestor of all members of the group but excludes some descendants of that ancestor; contrasts with monophyly and polyphyly.

Parasite An organism that lives physically on or in and at the expense of another organism.

Phylogeny (Gr. phylon, race, tribe, + geneia, origin). The origin and diversification of any taxon, or the evolutionary history of its origin and diversification, usually presented in the form of a dendrogram.

Phylogenetic tree A tree diagram whose branches represent current or past evolutionary lineages and which shows the hypothesized patterns of common descent among those lineages.

Phylum (Gr. phylon, race, tribe; pl. phyla) A chief category, between kingdom and class, of taxonomic classifications into which are grouped organisms of common descent that share a fundamental pattern of organization.

Polyphyly (Gr. polys, many, + phylon, tribe) The condition that a taxon or other group of organisms does not contain the most recent common ancestor of all members of the group, implying that it has multiple evolutionary origins; such groups are not valid as formal taxa and are recognized as such only through error. Contrasts with monophyly and paraphyly.

Radial cleavage Embryonic development in which early cleavage planes are symmetrical to the polar axis, each blastomere of one tier lying directly above the corresponding blastomere of the next layer; typically occurs with regulative cleavage.

Sister taxon (= sister group) The relationship between a pair of species or higher taxa that are each other’s closest phylogenetic relatives. As an example, humans (genus Homo) are sister taxon to the genus Pan (bonobos, chimpanzees), Sipuncula are sister taxon to the Annelida; Choanoflagellata are protozoa considered the most probable sister taxon of the Metazoa.

Species sing. and pl. (L. species, particular kind). A group of interbreeding individuals of common ancestry that are reproductively isolated from all other such groups; a taxonomic unit ranking below a genus and designated by a binomen consisting of its genus and the species name.

Spiral cleavage A type of embryonic cleavage in which cleavage planes (from the third one) are diagonal to the polar axis and unequal cells are produced by the alternate clockwise and counterclockwise cleavage around the axis of polarity; typically occurs with mosaic cleavage.

Taxon, pl. taxa (Gr. taxis, arrangement) Any taxonomic group or entity.

Taxonomic rank The Linnean category (kingdom, phylum, class, order, family, genus, species, and variations thereof) into which a recognized taxon is placed.

Taxonomy (Gr. taxis, arrangement, + nomos, law) Study of the principles of scientific classification; systematic ordering and naming of organisms.