在19世纪中叶，法国博物学家Jean Louis Armand de Quatrefages de Bréau将蠕虫分为孑孓——爬行和游泳是它们的核心内容，例如海洋岩虫（左图），和更为定栖、典型的在隧道中居住的非孑孓类——例如蚯蚓（右图）。
然而，早期的遗传学研究却对此种分类方法产生了质疑。
例如，这些研究表明，孑孓彼此之间的相互关系并非那么密切；相反，它们身体的相似性只是其适应类似生活方式以及环境的结果。
如今，更为全面的遗传学证据表明，de Bréau终究是正确的。
通过对来自34种环节蠕虫的231种基因进行的分析，德国奥斯纳布吕克大学的生物学家Torsten H. Struck和同事发现，孑孓和非孑孓类确实代表了两种不同的进化种群。
此外，研究小组发现，这两个种群之间的分离发生在蠕虫进化的很早期阶段；然而确切时间尚未搞清，这是因为蠕虫柔软的身体很难在化石中完好地保存下来。
为了在周围走动和觅食，孑孓保存了它们的刚毛附属物，然而非孑孓类则丧失了这些附属物，这是因为它们在进化后能够待在自己的洞穴中以沉积物和浮游生物为食。
研究人员在最近出版的《自然》杂志上报告了这一研究成果。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09864
Phylogenomic analyses unravel annelid evolution
Torsten H. Struck,1 Christiane Paul,2 Natascha Hill,3 Stefanie Hartmann,3 Christoph H?sel,1 Michael Kube,4 Bernhard Lieb,5 Achim Meyer,5 Ralph Tiedemann,2 Günter Purschke1 & Christoph Bleidorn2, 6
Annelida, the ringed worms, is a highly diverse animal phylum that includes more than 15,000 described species and constitutes the dominant benthic macrofauna from the intertidal zone down to the deep sea. A robust annelid phylogeny would shape our understanding of animal body-plan evolution and shed light on the bilaterian ground pattern. Traditionally, Annelida has been split into two major groups: Clitellata (earthworms and leeches) and polychaetes (bristle worms), but recent evidence suggests that other taxa that were once considered to be separate phyla (Sipuncula, Echiura and Siboglinidae (also known as Pogonophora)) should be included in Annelida1, 2, 3, 4. However, the deep-level evolutionary relationships of Annelida are still poorly understood, and a robust reconstruction of annelid evolutionary history is needed. Here we show that phylogenomic analyses of 34 annelid taxa, using 47,953 amino acid positions, recovered a well-supported phylogeny with strong support for major splits. Our results recover chaetopterids, myzostomids and sipunculids in the basal part of the tree, although the position of Myzostomida remains uncertain owing to its long branch. The remaining taxa are split into two clades: Errantia (which includes the model annelid Platynereis), and Sedentaria (which includes Clitellata). Ancestral character trait reconstructions indicate that these clades show adaptation to either an errant or a sedentary lifestyle, with alteration of accompanying morphological traits such as peristaltic movement, parapodia and sensory perception. Finally, life history characters in Annelida seem to be phylogenetically informative.
