The deuterostomes are a superphylum of animals that includes three phyla: the chordates (us!), the familiar echinoderms (sea urchins and starfish), and a peculiar group called the hemichordates (at times, you will see another phylum, the chaetognaths or arrow worms, grouped in the deuterostomes, but there is now evidence that they don't belong there). All are linked by their pattern of development. During gastrulation, animals form a structure called the blastopore, which is where migrating tissues tuck themselves inward to establish the three germ layers of the embryo. In deuterostomes, the blastopore will eventually become the anus. In the complementary category, the protostomes, which includes annelids and arthropods, the blastopore develops into the mouth.
The hemichordates are probably unfamiliar to most people reading this. They are marine worms that share two characteristics with us chordates: a hollow, dorsal nerve cord and a perforated feeding structure, the pharynx. They lack two others, the notochord and post-anal tail, hence the name hemichordate. There are two classes of hemichordate, the pterobranchs and the enteropneusts, which differ greatly in appearance and lifestyle.
Continue reading "Torquarator bullocki" (on Pharyngula)