The fine structure and histochemistry of the digestive diverticula of the protobranchiate bivalve Nucula sulcata

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Abstract
The digestive diverticula of Nucula sulcata consist of non-ciliated main ducts, ciliated secondary ducts and blind ending tubules. The cells lining the main ducts are of one type characterized by well-developed microvilli, iron-containing pigment spheres and, in animals fixed soon after collection, considerable amounts of lipid. The cells undergo a process of apo­crine secretion and it is possible that they are involved in the absorption and metabolism of lipids. The ciliated cells lining the secondary ducts exhibit features similar to those lining the main ducts. The epithelium lining the tubules consists of two cell types, pyramid-shaped, basiphilic cells and columnar, digestive cells. The basiphilic cells possess a single flagellum and a well-developed rough endoplasmic reticulum and Golgi apparatus; they appear to secrete a proteinaceous product. There is no evidence that they serve to replace the digestive cells. The digestive cells, as in other bivalves, are filled with membrane-bound vesicles and possess distinct Golgi elements characterized by the presence of intracisternal membranes. Feeding experiments using ferritin show that the membrane-bound vesicles constitute a lysosomal system within which exogenous material is digested. The morphological features and the histochemical properties of the pinosomes, heterophagosomes, heterolysosomes and residual bodies are described. Contrary to the results obtained from earlier work it is now clear that the digestive diverticula of Nucula serve an absorptive and digestive function. The mode of functioning of the diverticula is such, however, that only fluid and particles of macromolecular dimensions resulting from extracellular digestion in the gastric cavity are able to enter the diverticula. Peroxisomes occur in the basal regions of all the cell types; the appearance of the contained nucleoids differs in each cell type.

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