http://en.wikipedia.org/wiki/Pharyngeal_slit
Intriguingly, extantechinoderms lack pharyngeal structures, but fossil records reveal that ancestral forms of echinoderms had gill-like structures.Comparative developmental and genetic studies of these pharyngeal structures between hemichordates and urochordates have brought about important insights regarding the evolution of the deuterostome body plan.Comparative molecular biology has revealed that thePax1/9 genes (which encode for transcription factors) are expressed in similar patterns between hemichordates and urochordates; In vertebrates, Pax 1 and Pax 9 are expressed in the pharyngeal pouches and are important for thymus development.
Intriguingly, extantechinoderms lack pharyngeal structures, but fossil records reveal that ancestral forms of echinoderms had gill-like structures.Comparative developmental and genetic studies of these pharyngeal structures between hemichordates and urochordates have brought about important insights regarding the evolution of the deuterostome body plan.Comparative molecular biology has revealed that thePax1/9 genes (which encode for transcription factors) are expressed in similar patterns between hemichordates and urochordates; In vertebrates, Pax 1 and Pax 9 are expressed in the pharyngeal pouches and are important for thymus development.
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