软舌螺动物门

软舌螺动物（学名：Hyolitha）是生活在古生代的一类神秘动物，具有小圆锥形的螺壳。这些物种目前都已全部灭绝；其化石一般只能保存锥壳、口盖和附肢三个部分，外壳为钙质成分，两侧对称。根据形态学的分类，本物种是一类海生有壳的无脊椎动物，但再具体一点的分类众说纷纭：近期以软组织化石分析发现软舌螺动物具有触手冠 (Lophophore) 这种进食器官，指出软舌螺动物其实应该独立为一门，而且和包含腕足动物门在内的触手冠动物 (Lophophorata) 较相近。软舌螺动物的碳酸钙质外壳很大可能是由霰石组成。 有口盖（英语：operculum）保护，还有两条又名海伦体的弯弯附肢（helens）去支撑身体，以边缘增生（英语：marginal accretion）的形式成长。 体长一般在一公分到四公分，横切面呈三角形或椭圆形。部分物种有环纹或条纹。原状壳具有带横向束微结构的内层和包含纵向束的外层。本物种的附肢叫作“海伦体”， Helens are long structures that taper as they logarithmically coil gently in a ventral direction.The helens were calcareous, with an organic component, and had an organic-rich central core surrounded by concentric laminae of calcite. They grew by the addition of new material at their base, on the cavity side, leaving growth lines. They were originally described by Walcott as separate fossils under the genus name Helenia, (Walcott's wife was named Helena and his daughter Helen); Bruce Runnegar adopted the name helen when they were recognized as part of the hyolith organism.口盖（英语：operculum）紧紧的把壳关闭。 The operculum closes perfectly over the aperture of the shell, leaving two gaps through which the helens can protrude.It comprises two parts: the cardinal shield, a flat region at the top of the shell; and the conical shield, the bottom part, which is more conical.The inside of the shell bears a number of protrusions, notably the dorsal cardinal processes and the radially-arranged clavicles.人们依据有无口唇将软舌螺动物门动物分成两个目：Hyolitha have dorso-ventrally differentiated opercula, with the ventral surface of the shell extending forwards to form a shelf termed the ligula.The Orthothecida are somewhat more problematic, and probably contain a number of non-hyoliths simply because they are so difficult to identify with confidence, especially if their operculum is absent.They have a straight (planar) opening, sometimes with a notch on the bottom side, and sealed with an operculum that has no ligula, clavicles, furrow or rooflets.Hyptiotheca is an unusual hyolithid, in that it lacks clavicles.Orthothecids fall into two groups: one, the orcothecida sensu stricto,is kidney or heart shaped in cross-section due to a longitudinal groove on its ventral surface, and its opercula bear cardinal processes; the other has a rounded cross-section and often lacks cardinal processes, making them difficult to distinguish from other cornet-shaped calcareous organisms.All were sessile and benthic; some may have been filter feeders.Because hyoliths are extinct and do not obviously resemble any extant group, it is unclear which living group they are most closely related to. They may be molluscs; authors who suggest that they deserve their own phylum do not comment on the position of this phylum in the tree of life.Their grade of organization is considered to be of the 'mollusc-annelid-sipunculid' level,consistent with a Lophotrochozoan affinity, and comparison is usually drawn with the molluscs or sipunculids.Older studies (predating the Lophotrochozoan concept) consider hyoliths to represent a stem lineage of the clade containing (Mollusca + Annelida + Arthropoda).软舌螺可能是底栖生物。 寒武纪软舌螺是全球分布，没有迹象显示有地区差异，有迹象显示软舌螺有长寿命浮游幼虫生活期；但在奥陶系不同的组合变得越来越普遍。最早的软舌螺壳体化石大约出现在5亿4000万年前的西伯利亚 Purella antiqua Zone 的Nemakit-Daldynian阶，其类似物发现于中国Paragloborilus subglobosus–Purella squamulosa Zone 的 梅树村阶（寒武纪第一个阶）。。软舌螺动物门物种无论在其数量及其多样性都在寒武纪达至巅峰，之后逐步递减，直到灭绝。