File:American malacological bulletin (1988) (18156192775).jpg

Title: American malacological bulletin
Identifier: americanmal6719881990amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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SCHELTEMA: APLACOPHORA AND POLYPLACOPHORA 63 of continuous ribbon, each strip with rows of single denticulate teeth which are the mirror image of the opposed teeth (Figs. 7A, 8A, C). The two parts of the radular membrane are fused to a greater or lesser extent lengthwise along their medial (in- ner) edges forming a one-piece, unipartite radular ribbon along part of its length (Figs. 7B, 8A; Scheltema, 1981). The structure of the radula is clear only when it is dissected and isolated from surrounding tissue (Scheltema, 1981). Reconstructions from histologic sections have resulted
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Fig. 8. Radula of Simrothiella sp. b (undescribed), radular membrane indicated by stippling. A. Entire radula of a juvenile specimen, dor- sal view, anterior (oldest teeth) at top. Teeth of only left half of radula shown; teeth on the right are the mirror-image of those on the left. Denticles are added to the teeth medially as the radula widens and lengthens. B. Distal, oldest part of left radular strip shown folded under in A from ventral pharyngeal pocket; original, first-formed tooth is retained. C. Two views of the same two adjacent teeth from an adult specimen: upper teeth drawn in dorsal view as if they were on the right side of the radula, medial denticles on left; lower teeth from left side of radula drawn from beneath radular membrane. D. Most anterior part of the same adult radula from which teeth in C were drawn; com- parison with juvenile radula B indicates that there is dissolution at the distal end of the radula within the ventral pharyngeal pocket (Specimens from 2,633 m at 20°50'N, 109°06'W). Scale bars in mm. in misconceptions of actual structure and probable modifica- tions during its evolution (e.g. Nierstrasz, 1905; Salvini-Plawen, 1972, 1978 (Simrothiella), 1985). In order to differentiate the two states that exist for the radular membrane among mollusks, the terms "bipartite" and "unipartite" are used here, and the terms using '—stichous" are reserved for descriptions of the radular teeth only. Thus, a distichous radula can be either uni- or bipartite, but a monostichous radula is necessarily unipartite. The terms with '—serial," which should mean "arranged in series," are not used here, thus obviating the confusion of such a descrip- tion as "monoserial with paired teeth." As in other radulate Mollusca, the radular membrane in Aplacophora appears to migrate forward as teeth are add- ed by the odontoblasts; in most species the membranes turn anteroventrally into paired or unpaired ventral pharyngeal pockets, where dissolution of the radula apparently occurs (Figs. 7C, 8D). Unlike grazing gastropods and chitons, in all but one family of Aplacophora the teeth show no wear and thus do not rasp. The entire radula of juvenile specimen of Simrothiella (0.9 mm in length) has been examined. Within each ventral pharyngeal pocket is preserved the earliest ontogenetic development; the first tooth is a nondenticulate bar on a wide expanse of radular membrane (Fig. 8B). As the radula grows in length and width, denticles are added to the teeth medial- ly, i.e. at their inner edges (Fig. 8A). Histologic cross-sections through the proximal, blind end of the radular sac show odon- toblasts in two discrete groups, each presumably bound by basement membrane (Figs. 9, 10). The two groups lie within a single sac, surrounded in the usual manner by muscle. Within the Aplacophora, the radula has evolved at least twice from having a bipartite, distichous radula (Figs. 7, 8) to a radula with a unipartite radular membrane. In the Donder- siidae (Fig. 11), the radula is altogether absent or consists of

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