HICKMAN, C.S.: Comparative evolutionary, functional, and developmental morphology of the gastropod sensory epithelium

Detailed comparative structural and ultrastructural data provide an essential foundation for novel integrative syntheses of functional morphology, ecology, evolution, and development. Analysis of gastropod epithelial structures demonstrates the role of new data in fulfilling the promise of integrative biology. At the microanatomical level the elaborate sensory epithelium of trochoidean gastropods is more than a diverse array of cephalic, optic, and epipodial tentacles, cephalic lappets, neck lobes, epipodial fringes, sensory bursicles and micropapillae. Names previously assigned to these structures may have no functional or ecological implications, or they may suggest unconfirmed functions and dubious homologies. The most complex structural subunits are the micropapillae. Scanning and transmission electron microscopic analyses of papillae reveal several distinct forms of substructural organization. Sensory micropapillae of the cantharidine trochid Alcyna ocellata have an elaborate arrangement in which a single cell with microvillae wraps around 6 to 8 flattened and concentrically packed columnar sensory cells, each with a basal nucleus and as many as 12 distal cilia projecting from the tips. Each of the papillae is innervated by a branch of a central tentacular nerve. Comparable micropapillae are missing from homologous structures in a highly derived infaunal clade exemplified by the umboniine trochid Isanda coronata. Novel structures occur on a snout that is highly modified for episammic browsing, and the right neck lobe is modified into an inhalant siphon with unique marginal sensory papillae. A key feature in the evolution of development of the trochoidean epithelium is flexibility to change the position of functional microanatomical arrays relative to changes in ecology.