Body plan

This term, usually applied to animals, envisages a "blueprint" encompassing aspects such as symmetry, layers, segmentation, nerve, limb, and gut disposition.

Linnaeus's Vermes included effectively all other groups of animals, not only tapeworms, earthworms and leeches but molluscs, sea urchins and starfish, jellyfish, squid and cuttlefish.

Ernst Haeckel, in his 1866 Generelle Morphologie der Organismen, asserted that all living things were monophyletic (had a single evolutionary origin), being divided into plants, protista, and animals.

[7] Genes, embryos and development together determine the form of an adult organism's body, through the complex switching processes involved in morphogenesis.

A key discovery was the existence of groups of homeobox genes, which function as switches responsible for laying down the basic body plan in animals.

Modern groups of animals can be grouped by the arrangement of their body structures, so are said to possess different body plans.
Haeckel 's 'Monophyletischer Stambaum der Organismen' from Generelle Morphologie der Organismen (1866) with the three branches Plantae , Protista , Animalia