Enigmatic marine invertebrates known as penis worms are shedding new light on the evolution of digestion, while posing a curious embrionic question: which develops first, the mouth or the anus?

To produce the first description of the entire fetal development of penis worms, or priapulids, Swedish and Norwegian researchers looked at which genes are expressed during the development of the gut, the mouth and the anus in three-day-old worm embryos. In particular, they investigated embryos belonging to Priapus caudatus worms.

Living in shallow waters and measuring up eight inches long, these worms are “living fossils” that have changed very little since their priapulid ancestors thrived on the ocean floor in the Cambrian period some 500 million years ago.

Read more at Discovery.

Published On: November 1, 2012

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