The Animal kingdom appears suddenly in the fossil record over 600 My ago, in what has come to be called the Cambrian explosion. The boundary that defines the start of the Cambrian age is the appearance of fossil burrows that were obviously made by a complex animal, probably an arthropod—the name means “jointed feet”, think lobster. We will meet trilobites, a kind of arthropod, in a minute. Animals differ from plants in that they have a much more elaborate body plan that is built into the embryonic development. This plan is incorporated into the homeobox or "hox" genes and expression or development genes. The various animal phyla differ in the organization of these genes, and the resulting complexity of the species.