Functional Morphology of the Proboscis of Mitra Catalinae Dall 1920 (Mollusca: Gastropoda: Mitridae), and the Evolution of the Mitrid Epiproboscis
Several features of the proboscis morphology of Mitra catalinae Dall are unique among the known species of mitrid gastropods. These features include (1) a bilayered ring of comparatively large (7 μm high × 11 μm wide) squamous cells located midway along the length of the peristomial rim, (2) a pair of concentrically arranged cone-shaped buccal lips projecting into the peristomial rim, (3) a muscular esophagus lacking a well-developed secretory epithelium, and (4) a small pyriform epiproboscis consisting of longitudinal and transverse muscle fibers. It is proposed that the epiproboscis and the buccal lips regulate the movement of coelomic fluid and buoyant materials (e.g., eggs) from the sipunculan prey into the snail's alimentary tract by acting as a pump or as a valve. A hydrostatic mechansim appears to effect protraction of the epiproboscis. Comparison of the morphologies and functions of mitrid epiproboscides suggest that the epiproboscis developed from the musculature of the buccal mass, and the epipropboscis sheaths were derived from the walls of the buccal cavity. The mitrid epiproboscis shows structural and functional affinities with most of the other kinds of molluscan subradular organs. An evolutionary sequence of the mitrid epiproboscis is proposed in which the epiproboscis of M. catalinae is regarded as similar to a form ancestral to other modem mitrid epiproboscides.
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Document Type: Research Article
Publication date: 1991-05-01
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