Position of echinoderms in phylogeny of bilateria and establishment of D/V polarity during early development of the sea urchin embryo.

(A) Simplified phylogenetic tree of bilateria. Echinoderms together with hemichordates form a sister group of the chordates. (B) Scheme describing the early development of the sea urchin embryo together with the fate maps and gene expression territories. D/V polarity first becomes apparent at the beginning of gastrulation by the flattening of the presumptive ventral side, the bending of the gut, and the asymmetrical positioning of the skeletogenic mesenchyme cells that build the spicules. D/V polarity is further accentuated when four arms grow on the ventral side while the mouth opens on the ventral ectoderm and the larva lengthens along the dorsal side to become the pluteus larva.