The fine structure of epidermal papillae of Travisia forbesii (Annelida)

The structure of the epidermis of Travisia forbesii was described using light and electron microscopy. The epidermis is a highly modified variant of the normal one-layer polychaete epithelium. It consists of basal epidermal cells and an external layer of closely sited papillae consisting of glandular and supportive epidermal cells, and extensive electron-transparent intercellular spaces. The papillae are embedded in the thick cuticle. Each papilla has a peduncle, which is formed by one cell that penetrates the inner cuticle layer to the basal epidermal cells. A fold of basement membrane forms the core of the peduncle and ends in the base of a papilla. All epidermal cells are connected to each other with apical cell junctions and to the basement membrane with hemidesmosomes, so the epithelium is continuous and uninterrupted. The epidermis has an intra-epidermal neuron plexus. The structure of the papillae is compared with papillae and tubercles of other polychaetes, and the possible functional significance and phylogenetic implications of these structures are discussed.