 Animals were seen to develop into adults of widely differing body plan, often through similar stages, from the egg, but zoologists knew almost nothing about how embryonic development was controlled at the molecular level, and therefore equally little about how developmental processes had evolved.Charles Darwin argued that a shared embryonic structure implied a common ancestor.As an example of this, Darwin cited in his 1859 book "On the Origin of Species" the shrimp-like larva of the barnacle, whose sessile adults looked nothing like other arthropods; Linnaeus and Cuvier had classified them as molluscs.