The Digestive Gland

The digestive lobules possess both digestive and excretory cells. The digestive cells (Fig. 13, DI.C.) are tall (100μ) and columnar, with basal nuclei; their cytoplasm is crowded with spherules, formed of greenish-yellow particles clumped together, in little boluses, 7μ across. From time to time these are egested from the cells and returned to the stomach. As in some other prosobranchs, the free cell surface is probably sparsely ciliated in life, but cilia are difficult to identify in sections, and the cell borders take on a convex pseudopodial appearance. Excretory cells (EXC.C.), presumably a means of rejecting absorbed chlorophyllous pigments, occur much less frequently, interspersed with the digestive cells. They are broad-based and pyramidal with brown-staining contents, and usually contain each a single brownish-black spherule, 25μ or more across. These spherules appear with the digestive cell contents in the egested material passed into the stomach. Faecal material is derived from two sources, first from the empty diatom frustules which are never found within the diverticula and apparently after preliminary digestion of their contents are carried directly to the proximal intestine, secondly from the egested particles from the two types of digestive gland cell. As in Struthiolaria, but contrary to the suggestion of Mansour (1946) in certain lamellibranchs, the greenish digestive cell particles are not enzymatic but represent waste products of digestion. In Vermicularia they were actually observed in the living stomach to pass at once by ciliary currents from the anterior diverticulum to the proximal intestine.