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.