The current system of animal classification relies on the number and s
tructure of the germ layers built by early embryos. This system dates
back to the beginning of the XXth century. In spite of the increasing
influences exerted by phylogenetic studies on taxonomy, there has been
no fundamental modification of the development-based system of classi
fication. This is not surprising. A certain parallelism exists between
development and evolution. When related animals are compared, more si
milarities can be detected between their embryonic forms that between
their adult ones. Like animals have late diverging developments; unlik
e animals have early diverging developments. A similar trend can be di
scerned in evolution. Closely related groups have a recent origin; dis
tantly related groups have a more ancient origin. Therefore, a develop
ment-based taxonomy is likely to reflect phylogeny.