Evolution of two novel feeding strategies among caenogastropod molluscs, suspension feeding in calyptraeids such as Crepidula fornicata and predatory feeding with a pleurembolic proboscis among neogastropods, may have both involved elongation of the anterior esophagus. Emergence of predatory feeding with a proboscis is particularly significant because it correlates with the rapid adaptive radiation of buccinoidean and muricoidean neogastropods during the Cretaceous. However, the notion that this important evolutionary transition involved elongation of the anterior esophagus to extend down a long proboscis has been disputed by evidence that it may have been the wall of the buccal cavity that elongated. We undertook a comparative study on foregut morphogenesis during larval and metamorphic development in C. fornicata and in three species of neogastropods with a pleurembolic proboscis to examine the hypothesis that the same region of foregut has elongated in all. We approached this by identifying a conserved marker for the boundary between buccal cavity and anterior esophagus, which was recognizable before the developing foregut showed regional differences in length. A survey of four species of littorinimorph caenogastropods suggested that the site of neurogenic placodes for the buccal ganglia could serve as this marker. Results showed that foregut lengthening in C. fornicata involved elongation posterior to neurogenic placodes for buccal ganglia, an area that corresponded to the anterior esophagus in the other littorinimorphs. However, foregut elongation occurred anterior to neurogenic placodes for buccal ganglia in two buccinoidean and one muricoidean neogastropod. The elongated foregut within the pleurembolic proboscis of these neogastropods qualifies as anterior esophagus only if the definition of the anterior esophagus is expanded to include the dorsal folds that run down the roof of the buccal cavity. Regardless of how the anterior esophagus is defined, comparative developmental data do not support the hypothesis of homology between the elongated adult foregut regions in C. fornicata and in neogastropods with a pleurembolic proboscis.

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软体动物前肠的同源性难题：从比较发育模式看

Caenogastropod 软体动物中两种新型摄食策略的进化，在 calyptraeidula fornicata 中的悬浮摄食和在新腹足类中使用胸膜长鼻的捕食性摄食，可能都涉及前食管的伸长。长鼻捕食性食物的出现特别重要，因为它与白垩纪 buccinoidean 和 muricoidean 新腹足类动物的快速适应性辐射相关。然而，这一重要的进化转变涉及前食道伸长以向下延伸到长长的长鼻的观点受到争议，有证据表明可能是口腔壁被拉长了。我们对 C. 幼虫和变质发育过程中的前肠形态发生进行了比较研究。fornicata 和三种具有胸膜长鼻的新腹足类动物，以检查前肠的同一区域已全部拉长的假设。我们通过确定口腔和前食管之间边界的保守标记来解决这个问题，在发育中的前肠显示区域长度差异之前可以识别该标记。对四种 littorinimorph caenogastropods 的调查表明，颊神经节的神经源性基板的位置可以作为这个标记。结果表明，C. fornicata 中的前肠延长涉及颊神经节的神经源性基板后方的延长，该区域对应于其他 littorinimorphs 中的前食道。然而，前肠伸长发生在两个 buccinoidean 和一个 muricoidean 新腹足动物的颊神经节的神经源性基板之前。只有将前食管的定义扩大到包括沿颊腔顶部向下延伸的背皱襞，这些新腹足类动物胸膜长鼻内的细长前肠才有资格称为前食管。不管前食道是如何定义的，比较发育数据不支持毛囊长长的成年前肠区域与具有胸膜长鼻的新腹足类动物之间的同源性假设。只有将前食管的定义扩大到包括沿颊腔顶部向下延伸的背皱襞，这些新腹足类动物胸膜长鼻内的细长前肠才有资格称为前食管。不管前食道是如何定义的，比较发育数据都不支持毛囊长长的成人前肠区域和具有胸膜长鼻的新腹足类动物之间的同源性假设。只有将前食管的定义扩大到包括沿颊腔顶部向下延伸的背皱襞，这些新腹足类动物胸膜长鼻内的细长前肠才有资格称为前食管。不管前食道是如何定义的，比较发育数据不支持毛囊长长的成年前肠区域与具有胸膜长鼻的新腹足类动物之间的同源性假设。