FINE-STRUCTURE OF CECA AND MANTLE OF ARCOID AND LIMOPSOID BIVALVES (MOLLUSCA, PTERIOMORPHA)

Shell pores with cellular content (caeca) are present in various mollu scan groups. Up to now, the functions of the caeca are speculative, an d also homology is questionable. To clarify the caeca of bivalve shell pores (tubules), the ultrastructure of the mantle of several species of arcoid and limopsoid bivalves has been investigated using microwave supported fixation and transmission electron microscopy. In general, the outer mantle epithelium is composed of several cell types: caecal cells have processes that extend to the periostracum; supporting cells have prominent tonofilaments that attach the mantle to the shell, man y mucous cells, and enigmatic microvillar pads that may have a photore ceptive function. The hemocoel is vascularized and contains cells that are typical of the bivalve hemocoel such as amoebocytes, rhogocytes ( pore-cells), muscle and nerve fibers, and gliointerstitial cells. The inner mantle epithelium is quite homogeneously composed of a partly ci liated, squamous epithelium with a brush border suggesting metabolic a nd respiratory functions. Structure (monocellular) and mode of develop ment (secondary etching) contradict speculation concerning homology of bivalve caeca with those of other molluscan groups.