The term "body plan" refers to the general similarities in development and form and function among members of a particular phylum. Another name for these similarities is baüplan, which is the German word for "body plan."

A body plan is a group of structural and developmental characteristics that can be used to identify a group of animals, such as a phylum. All members of a particular group share the same body plan at some point during their development—in the embryonic, larval, or adult stage. Biologists have long observed that anatomy and embryology reflect shared underlying structural plans. These plans can be used to define taxonomic groups (usually phyla) and to construct hierarchical classifications within groups (organisms with similar body plans tend to be more closely related).

Similarities and differences in adult shape and form, as well as the developmental pattern of embryos, provide the...