You aren't what you eat

摘要

Only those fond of the less-frequented pages of zoology textbooks ―the chapters that deal with organisms such as gastrotrichs and gnathostomulids―will have ever heard of a small marine worm called Xenoturbella bocki. First described in 1949, this creature has struggled to find a phylogenetic home, and has been loosely attached to many animal groups. The problem is its unassuming nature. Sans proper gonads, excretory system, body cavity, even a through gut, the worm has very few distinguishing features. And it is only about 2 cm in length. In the end, zoologists usually admitted defeat and banished it to one of several groups of primitive flatworm. In 1997, however, it was announced that Xenoturbella is, in fact, a mollusc, and not only that, a bivalve mollusc, even if, like the persona of Eliza Doolittle in My Fair Lady, neither Hungarian nor of royal blood. DNA from the creature closely resembled that of the small bivalve Nucula, and―this was the clincher―mollusc-like eggs were found inside Xenoturbella specimens. It was thought that the animal started life as a conventional mollusc, but that by the time it reached adulthood it had for some reason shed virtually everything that would mark it as such.