Development
The hemichordates are the closest extant phylogenetic relatives of chordates among the invertebrates. Thus these marine worms, described to be the sister group of the echinoderms (such as sea urchins), are of great interest for the study of the origins of chordate development. There are several species of hemichordates, with a moderate diversity of embryological development among these species. Hemichordates are classically known to develop in two ways, both directly and indirectly. Hemichordates are a phylum composed of two classes, the enteropneusts and the pterobranchs, both being forms of marine worm.
The enteropneusts have two developmental strategies: direct and indirect development. The indirect way of development is known to end in an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in a larval stage that feeds on plankton before turning into an adult worm. Those species that are direct developing bypass this prolonged larval stage and develop directly into an adult worm.
The development of pterobranchs is studied only in Rhabdopleurafrom Plymouth, England and from Bermuda.
The following details the development of two popularly studied species of the hemichordata phylum Saccoglossus kowalevskii and Ptychodera flava. Saccoglossus kowalevskii is a direct developer and Ptychodera flava is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly.
Read more about this topic: Hemichordates
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