*University of Illinois, Urbana, IL, 601 S. Goodwin, Avenue. Urbana, IL 61801, USA; University of Hawaii, Kewalo Marine Laboratory, 41 Ahui Street, Honolulu HI 96813, USA.
Integr Comp Biol. 2007 Dec;47(6):865-71. doi: 10.1093/icb/icm035. Epub 2007 May 24.
A number of hypotheses have been presented regarding the origins of the metazoans and, more specifically, the Bilateria. Using various phylogenetic analyses, characteristics have been mapped on phylogenetic trees to infer ancestral body plans and life history strategies of those ancestors. Many arguments on the evolution of the Bilateria are based on the presumed homology of certain characteristics of extant larva and adults, including various ciliated bands involved in feeding and locomotion. This article considers a recent study indicating that the second, downstream-collecting, ciliated band in the veliger larva of the gastropod mollusc, Crepidula fornicata, is actually derived from secondary trochoblasts (derived from second quartet micromeres), that normally form part of the prototrochal band found in other spiralian phyla (Hejnol et al. 2007). Despite previous arguments, these new findings suggest that the second ciliated band in the veliger larva is not homologous to the metatroch found in the trochophore larva of some other spiralians, such as the annelid, Polygordius lacteus. In the latter case, the metatroch was reported to be formed by a different set of lineage precursors (derived from third quartet micromeres) (Woltereck 1904). These findings have important implications for the interpretation of various hypotheses related to the evolution of metazoan phyla.
已经提出了一些关于后生动物起源的假说，更具体地说，是关于两侧对称动物的起源。通过各种系统发育分析，将特征映射到系统发育树上，以推断那些祖先的原始身体计划和生活史策略。关于两侧对称动物进化的许多论点都是基于对现存幼虫和成虫某些特征的假定同源性，包括参与进食和运动的各种纤毛带。本文考虑了一项最近的研究，该研究表明，腹足纲软体动物 Crepida fornicata 的担轮幼虫中的第二个、下游收集的纤毛带实际上是由次生担轮细胞（来自第二四分体小卵裂球）衍生而来的，这些细胞通常构成其他螺旋体门中发现的原担轮带的一部分（Hejnol 等人，2007 年）。尽管存在先前的论点，但这些新发现表明，担轮幼虫中的第二个纤毛带与某些其他螺旋体的担轮幼虫中的原担轮不是同源的，例如环节动物 Polygordius lacteus。在后一种情况下，原担轮被报道是由不同的谱系前体（来自第三四分体小卵裂球）形成的（Woltereck，1904 年）。这些发现对解释与后生动物门进化相关的各种假说具有重要意义。
