ULTRASTRUCTURE OF THE DIGESTIVE GLAND IN THE OPISTHOBRANCH MOLLUSK, RUNCINA

In the digestive gland of two Runcina species (R. coronata and R. ferr uginea), four cell types have been identified: digestive cells, microt ubule-containing cells, secretory cells, and mineral-containing granul e cells. The digestive cells are the most numerous. Their apical brush border and the endocytotic vesicles indicate intensive uptake of food particles. A complex lysosomal system is responsible for breakdown, s torage, and later expulsion of the degraded material into the gland lu men. The microtubule-containing cells exhibit voluminous endoplasmic r eticulum cisternae filled with microtubules. Such microtubule-containi ng vacuoles have been described in ectodermal cells of Runcina and in other cephalaspids but not in the digestive gland. Their function is n ot known. The third cell type represents secretory cells. They produce large, spherical electron-dense secretory granules, which probably ta ke part in extracellular digestion. Cells with birefringent, concentri cally structured mineral-containing granules in their cytoplasm, seem typical for the digestive gland in most gastropods. The mineral compos ition varies among different species. In Runcina, the x-ray microanaly tical spectrum reveals magnesium phosphate as the main component.