As reported by earlier investigators, the epithelium of the digestive tubules is composed of two cell-types. One type of cell is glandular, the other type is absorptive and digestive, and to a lesser extent secretory. The latter type of cell also contains glycogen and numerous lipid globules, so that the digestive gland as a whole contains a large quantity of reserve food material. The epithelium of the digestive duct possesses a single cell-type; the cells are ciliated and heavily pigmented, and they produce a viscous secretion.
The salivary gland is a compound tubular gland. The cells elaborate a secretion containing protein and probably some carbohydrate.