Acoels are among the simplest worms and therefore have often been pivotal in discussions of the origin of the Bilateria. Initially thought primitive because of their “planula-like” morphology, including their lumenless digestive system, they were subsequently dismissed by many morphologists as a specialized clade of the Platyhelminthes. However, since molecular phylogenies placed them outside the Platyhelminthes and outside all other phyla at the base of the Bilateria, they became the focus of renewed debate and research. We review what is currently known of acoels, including information regarding their morphology, development, systematics, and phylogenetic relationships, and put some of these topics in a historical perspective to show how the application of new methods contributed to the progress in understanding these animals. Taking all available data into consideration, clear-cut conclusions cannot be made; however, in our view it becomes successively clearer that acoelomorphs are a “basal” but “divergent” branch of the Bilateria.

中文翻译：

---

Acoela：关于它们的种类和亲属关系，尤其是与线虫和异种涡虫（Bilateria incertae sedis）

Acoels 是最简单的蠕虫之一，因此在讨论 Bilateria 的起源时经常是关键。最初被认为是原始的，因为它们的“类扁平”形态，包括它们的无腔消化系统，随后被许多形态学家认为是扁形动物的一个特殊进化枝。然而，由于分子系统发育将它们置于扁形动物门之外和位于双侧动物底部的所有其他门之外，它们成为重新辩论和研究的焦点。我们回顾了目前对无腔动物的了解，包括关于它们的形态、发育、系统学和系统发育关系的信息，并将其中一些主题放在历史的角度，以展示新方法的应用如何有助于理解这些动物的进展。考虑到所有可用数据，无法得出明确的结论；然而，在我们看来，越来越清楚的是，腔形动物是比拉特里亚的“基础”但“发散”的分支。