软舌螺动物门

软舌螺动物的碳酸鈣質外殼很大可能是由霰石組成[5]。 有口盖保護，還有兩條又名海倫體的彎彎附肢（）去支撐身體，以邊緣增生的形式成長[5]。 體長一般在一公分到四公分，橫切面呈三角形或橢圓形。部份物種有環紋或條紋。

Hyoliths from the Middle Ordovician of northern Estonia; these are internal molds.

人们依据有无口唇将软舌螺动物门动物分成两个目：

• †有唇软舌螺目 Hyolithida
• †直管螺目 Orthothecida

Hyolitha have dorso-ventrally differentiated opercula, with the ventral surface of the shell extending forwards to form a shelf termed the ligula.[5]

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.[5]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.[6]

Hyptiotheca is an unusual hyolithid, in that it lacks clavicles.[6]

Orthothecids fall into two groups: one, the orcothecida sensu stricto,[7]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.[6]All were sessile and benthic; some may have been filter feeders.[7]

Haplophrentis carinatus from the Stephen Formation, Burgess Shale (Middle Cambrian), Burgess Pass, British Columbia, Canada.

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.[8]Their grade of organization is considered to be of the 'mollusc-annelid-sipunculid' level,[9]consistent with a Lophotrochozoan affinity, and comparison is usually drawn with the molluscs or sipunculids.[9][10]Older studies (predating the Lophotrochozoan concept) consider hyoliths to represent a stem lineage of the clade containing (Mollusca + Annelida + Arthropoda).[3]

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。[6]

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）。[11]。软舌螺动物门物種無論在其數量及其多樣性都在寒武紀達至巔峰，之後逐步遞減，直到滅絕。[2][12]