Adult specimen of "Darwin's coral" and ontogenesis of Haeckel's model system. Morphology of the coral Monoxenia darwinii (A), an organism discovered by Ernst Haeckel in 1873 in the Red Sea and later described by him as a new species, named in honour of Charles Darwin. The development of M. darwinii is shown in (B), from the fertilized egg (A/B) to the so-called "Becherlarve" (beaker larvae) or gastrula (K/I). Haeckel coined the term "Gastraea" to denote this phylogenetically conserved stage in animal development [adapted from ref. 27].