American malacological bulletin (1988) (18156187425)

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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62 AMER. MALAC. BULL. 6(1) (1988) RADULA APLACOPHORAN RADULA Evidence from the radula morphology of aplacophor- ans and from the ontogeny of gastropod and chiton radulae suggests that the molluscan radula orginated as a paired structure. The radula in chitons, the monoplacophoran Neopilina, gastropods, and scaphopods is a chitinous structure formed of a single continuous ribbon, or radular membrane, which bears serial rows of teeth; both ribbon and teeth are continual- ly secreted at the proximal end of a pharyngeal diverticulum, the radular sac (Fretter and Graham, 1962; Kerth, 1983; Scheltema, unpub. data). Each row of teeth has left and right sides and usually a central, or median, tooth. The radula is bilaterally symmetrical around the central tooth, that is, the teeth of each side are mirror images of one another. Along the length of the ribbon each tooth has the same shape as the tooth in front of and behind it, that is, the rows of teeth are serially repeated. In the Aplacophora, the radula is formed in the usual manner and is likewise bilaterally symmetrical and serially repeated (Figs. 7A, 8A, C). The radula has been called monostichous or monoserial if there is only a single tooth in a row; with two mirror-image teeth in each row, distichous or biserial; and with more than two mirror-image teeth, polystichous or polyserial (Nierstrasz, 1905). The usual type of radula in the Aplacophora is distichous; a central tooth is lacking in nearly all species. Uni- que among mollusks the radular membrane itself is divided down the middle so that the entire radula is a bipartite, bilaterally symmetrical, serial structure consisting of two strips

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1988

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