File:American malacological bulletin (1988) (18156402885).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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158 AMER. MALAC. BULL. 6(1) (1988)
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Fig. 11. Photoreceptor cell at the transition of perikaryon and rhabdomere (mvb, multivesicular body; nu, nucleus; rh, rhabdomere; ser, specialised agranular ER). Fig. 12. Longitudinal section of the "arm" part of a micraesthete cell (mit, mitochondrion; mt, microtubules). Fig. 13. Sub- sidiary cap (sc) with the microvilli (mv) of the underlying micraesthete cell. Fig. 14. Longitudinal section through an aesthete canal (nu, nucleus of a peripheral cell). Arrows indicate profiles resembling neurites. found near the base, but their relationship to the central cells is not clear. A possible function could be to detect desicca- tion and/or animals grazing on the shell (and eating also the distal parts of the aesthetes), with a subsequent reaction of the secretory cells. Lepidopleurus cajetanus has the best developed central cells of all species studied so far; in the Lepidopleurina the articulamentum is lacking in broad shell areas and the aesthetes are connected directly with the dor- sal epithelium. Destruction of the aesthetes and a subsequent invasion of microorganisms into empty aesthete canals could be much more severe in this group than in the Ischnochitonina or the Acanthochitonina. The photoreceptor cells resemble in detail the photo- receptor cells of Chiton olivaceus and Acanthochitona fascicularis (Fischer, 1978b, 1979; Fischer and Renner, 1978) as well as the photoreceptor cells in the two different shell- eye types in the chitons (Boyle, 1969; Haas and Kriesten, 1978). Boyle (1974) found no typical photoreceptor cells in the aesthetes of Lepidochitona cinerea L., but he described "microvillous areas" and areas with "lamellate bodies". These very likely correspond to the rhabdomere and the agranular ER of the photoreceptor cells. This cell type is the only one in the aesthete that shows ultrastructural differences be- tween light- and dark-adapted animals (Fischer, 1978a). Ad- ditionally, experiments with partially masked C. olivaceus clearly show that the shell valves contain photoreceptive elements (Fischer, unpub. data). As a common feature, photoreceptor cells seem to be a primary part in chiton aesthetes. At first sight, it is astonishing that the shell valves con- tain so many, and simple, photoreceptive elements. In some species, aesthetes in certain shell areas have been transform- ed into eyes of various complexity (Boyle, 1969; Fischer, 1978a; Haas and Kriesten, 1978). Most species, however, have only the "normal" aesthete type. The situation could be com- parable with other invertebrates, like the earthworm which avoid light during the day and feed at dawn and when it is dark. The earthworm also has many primitive light receptors dispersed in the skin. As behavioural studies show (Fischer, unpub. data), the photoreceptor cells in the aesthetes have a similar function. Most chiton species avoid bright sunlight and hide below stones or in the mud during the day. Chitons with masked shells do not exhibit such a behaviour (except species that also have photoreceptor cells in their girdle

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