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Department of Biology, University of Victoria, British Columbia, Canada V8W 3N5.
J Morphol. 2002 May;252(2):202-17. doi: 10.1002/jmor.1099.
The specialized, postmetamorphic feeding structures of predatory caenogastropods evolved by changes to an ancestral caenogastropod developmental program that generated a planktotrophic larval stage followed by a herbivorous postmetamorphic stage. As part of a program of comparative studies aimed at reconstructing these developmental changes, I studied the development of the postmetamorphic feeding system of Marsenina stearnsii using histological sections for light microscopy and scanning and transmission electron microscopy. The feeding system of this species has two very different designs during ontogeny. The larval system uses ciliary effectors to capture and ingest microalgae, whereas the juvenile/adult system includes a proboscis, jaws, and radular apparatus for predation on ascidian zooids. The postmetamorphic foregut begins to develop during the early larval phase, but the anlagen does not interfere with larval feeding because it develops as an increasingly elaborate outpocketing from the ventral wall of the larval esophagus. At metamorphosis, an opening is created in the anterior tip of the prospective, postmetamorphic buccal cavity and the margins of this opening anneal with the metamorphically remodeled lips of the larval mouth. This process exposes the jaws, which differentiate within the buccal cavity prior to metamorphosis. As a working hypothesis, I suggest that rupture of the buccal cavity to the outside at metamorphosis was selected as a mechanism to allow precocious development of jaws in species where jaws enhanced feeding performance by young juveniles. The larval esophagus of M. stearnsii appears to be completely destroyed at metamorphosis. Larval esophageal cells have distinctive apical characteristics (cilia, blebbed microvilli, stacks of lamellae within the glycocalyx) and no cells having this signature persist through metamorphosis. Development of the proboscis and proboscis sac, which begins prior to metamorphosis, conforms to previous descriptions of pleurembolic proboscis development, although an acrembolic proboscis has been ascribed to members of the Lamellaroidea.
掠食性新腹足类动物特化的变态后摄食结构是通过对祖先新腹足类动物发育程序的改变而进化而来的，该发育程序产生了浮游生物食性的幼虫阶段，随后是草食性的变态后阶段。作为旨在重建这些发育变化的比较研究计划的一部分，我使用用于光学显微镜、扫描电子显微镜和透射电子显微镜的组织切片，研究了斯特恩斯玛森螺变态后摄食系统的发育。该物种的摄食系统在个体发育过程中有两种非常不同的设计。幼虫系统使用纤毛效应器捕获和摄取微藻，而幼体/成体系统包括用于捕食海鞘类动物体的吻、颚和齿舌器官。变态后的前肠在幼虫早期开始发育，但原基不会干扰幼虫摄食，因为它是从幼虫食道腹壁逐渐发育出的一个越来越精细的袋状结构。在变态时，在未来变态后口腔的前端形成一个开口，这个开口的边缘与幼虫口经过变态重塑的唇部愈合。这个过程暴露了颚，颚在变态前在口腔内分化。作为一个工作假设，我认为在变态时口腔向外部破裂被选择为一种机制，以允许在颚能提高幼体摄食性能的物种中颚提前发育。斯特恩斯玛森螺的幼虫食道在变态时似乎被完全破坏。幼虫食道细胞具有独特的顶端特征（纤毛、泡状微绒毛、糖萼内的片层堆叠），并且没有具有这种特征的细胞在变态过程中持续存在。吻和吻囊的发育在变态前就开始了，符合之前对侧卷式吻发育的描述，尽管曾有观点认为卷式吻见于薄片螺超科的成员。
