File:American malacological bulletin (1988) (18129919896).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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FISCHER: AESTHETES IN LEPIDOPLEURUS CAJETANUS 157
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body of the aesthete as a sheet about 0.75 in width. They are not present in all parts of an aesthete (e.g. Fig. 7). The fine structure varies considerably, e.g. in the content of decom- posing structures. In the basal part of the aesthete, peripheral cells form an extracellular sheet of fine filaments into the shell material. Some of the filaments protrude, roughly perpen- dicularly, far into the shell substance. AESTHETE CANAL. The aesthete canal is surrounded by peripheral cells (Fig. 14). In the center, various fibers (basal extensions of the aesthete cells to the epithelium) run towards the epithelium under or lateral to the shell valves. Some of the fibers connect the secretory cells with the epithelium; these fibers contain mitochondria and microtubules. About ten of the fibers are much thinner than those of the secretory cells (0.4 versus 1.5 /*m). They are densely filled with microtubules and are probably nervous elements (Fig. 14). Structures resembling neurosecretory elements are also a regular feature in this area. DISCUSSION The general structure of the aesthetes of Lepidopleurus cajetanus is very similar to that of previously studied species. Despite the differences in the architecture of the shell valves, there is no major difference between the aesthetes in all three extant polyplacophoran suborders. The aesthetes are obvious- ly an evolutionarily old system in the chitons. The different cell types, each with pronounced ultra- structural characteristics, suggest that the aesthetes are com- pound organs with both a sensory and a secretory function. It is not clear whether some or all cell types work together to perform a more complex function or whether they function more or less independently. The secretory cells produce secretions basally and release them apically. In Chiton olivaceus, animals outside the water show an increased secretory activity (Fischer, 1978a). Additionally, recordings with a glass microelectrode show slow rhythmic changes in the electrical potential under the same conditions (Fischer, unpub. data). This could sug- gest that one function of the secretion is to prevent the desi- ccation of the aesthetes during low tide. However, species that prefer to live in deeper water also have well developed secretory cells (Baxter ef a/., 1987). The secretions probably have other protective functions, e.g. against predators or organisms growing on the shell. One indication for such a function is that the pores of the apical cap open only in older aesthetes, whereas newly formed aesthetes are covered by the periostracum. The role of the central cells is not known. In their fine structure they resemble chemoreceptors of insects (Ernst, 1969). The position of this cell type underneath the perforated apical cap, the pronounced membrane proliferations (microvilli and cilia), and the high metabolic activity support such a hypothesis. Structures resembling nervous elements were

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