“阿克爾扁形蟲(chóng)”(Acoel flatworms)是最簡(jiǎn)單的動(dòng)物之一，它們是如此地簡(jiǎn)單,，以至于既沒(méi)有一個(gè)貫通的腸道,，又沒(méi)有一個(gè)體腔。但新的分子研究將它們從其在動(dòng)物進(jìn)化的底部位置挪開(kāi)了,，將其與棘皮類動(dòng)物（海星,、海膽等等）放在了一起,，并大大拉近了它們與脊索動(dòng)物（包括人類在內(nèi)的一個(gè)類群）的距離,。在這之前，人們?cè)l(fā)現(xiàn),，“異渦蟲(chóng)”(Xenoturbella)（一種有神秘演化關(guān)系的簡(jiǎn)單扁形蟲(chóng)）也屬于這一類群,。這項(xiàng)研究說(shuō)明，“阿克爾”并不是像人們以前所以為的那樣從一開(kāi)始就很簡(jiǎn)單,，而是隨著時(shí)間推移而變簡(jiǎn)單的,，失去了如體腔、肛門(mén)和鰓裂等特征,。（生物谷Bioon.com）
生物谷推薦原文出處：
Nature doi:10.1038/nature09676
Acoelomorph flatworms are deuterostomes related to Xenoturbella
Hervé Philippe,1 Henner Brinkmann,1 Richard R. Copley,2 Leonid L. Moroz,3 Hiroaki Nakano,4, 9 Albert J. Poustka,5 Andreas Wallberg,6 Kevin J. Peterson7 & Maximilian J. Telford8
Xenoturbellida and Acoelomorpha are marine worms with contentious ancestry. Both were originally associated with the flatworms (Platyhelminthes), but molecular data have revised their phylogenetic positions, generally linking Xenoturbellida to the deuterostomes1, 2 and positioning the Acoelomorpha as the most basally branching bilaterian group(s)3, 4, 5, 6. Recent phylogenomic data suggested that Xenoturbellida and Acoelomorpha are sister taxa and together constitute an early branch of Bilateria7. Here we assemble three independent data sets—mitochondrial genes, a phylogenomic data set of 38,330 amino-acid positions and new microRNA (miRNA) complements—and show that the position of Acoelomorpha is strongly affected by a long-branch attraction (LBA) artefact. When we minimize LBA we find consistent support for a position of both acoelomorphs and Xenoturbella within the deuterostomes. The most likely phylogeny links Xenoturbella and Acoelomorpha in a clade we call Xenacoelomorpha. The Xenacoelomorpha is the sister group of the Ambulacraria (hemichordates and echinoderms). We show that analyses of miRNA complements8 have been affected by character loss in the acoels and that both groups possess one miRNA and the gene Rsb66 otherwise specific to deuterostomes. In addition, Xenoturbella shares one miRNA with the ambulacrarians, and two with the acoels. This phylogeny makes sense of the shared characteristics of Xenoturbellida and Acoelomorpha, such as ciliary ultrastructure and diffuse nervous system, and implies the loss of various deuterostome characters in the Xenacoelomorpha including coelomic cavities, through gut and gill slits.
