SEM — Observations on the Shell Plates of Three Polyplacophorans (Mollusca, Amphineura) by F. P. Fischer and M. Renner'') Zoologisches Institut der Universität München Abstract The shell surface of the Polyplacophorans Chiton olivaceus, Lepidopleurus cajetanns, Acanthochiton fascicularis was studied by scanning electron microscopy. In the latter two species the aesthete caps terminate at projections of the shell; in A. fascicularis these are especially elaborate. The apical caps appear perforated in all three species. There are also pores in the subsidiary caps, but these don't open to shell surface in contrast to the apical caps. The nature of these pores is investigated. Key-words: Polyplacophora — Shell surface — Aesthete Organs — Scanning electron microscopy Introduction The outer shell layer of the dorsal valves of the Polyplacophora (the tegmentum) contains a large number of small organs — the aesthetes — which are thought to have some sort of sensory function. Each of these aesthetes consists of a shaft, the megalaesthete, from which small boughs, the micraesthetes, brauch off to the shell surface and terminate there with the subsidiary caps. The megalaesthete forms there the apical cap. Normally one aesthete has thirty to sixty cells of various types. Their morphology is known in a few species (Plate 1898/1902, Nowikoff 1907, Knorre 1925), the fine structure has been studied by Boyle (1974) in Lepido-chitona cinereus. The same author also investigated the shell surfaces of four species of the Order Ischnochitonina (1976). Although there are several studies on the function of these organs (Arey & Crozier 1919, OmeLich 1967, Boyle 1972), this question is not quite solved yet; "■) We wish to thank Prof. Dr. R. Wetzstein, Anatomische Anstalt der Universität Mün-chen, for the SEM-facilities, which are subsidized by the DFG, and Mr. Ch. Grosse, Ing. PTA, for help at the scanning electron microscope. 49
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