LIGHT AND ELECTRON-MICROSCOPIC INVESTIGATIONS ON SHELL PORES (CECA) OF FISSURELLID LIMPETS (MOLLUSCA, ARCHAEOGASTROPODA)

The organic content (caecum) of the shell pores of five species of fis surellid (key-hole) limpets was investigated by means of light and ele ctron microscopy. Fissurellid caeca are unbranched, polycellular exten sions of the mantle epithelium, which penetrate the whole shell and co ntact the periostracum via organic fibres called 'brush'. They are bui lt up by two cell-types, which enclose a central lumen, and are formed concurrently with the shell itself. Because of fundamental structural and developmental differences, the superficially similar shell pores of bivalves (caeca) and polyplacophorans (aesthetes) are considered as analogous rather than homologous organs. Similarities are found with brachiopod caeca, probably owing to similar functions.