A HISTOLOGICAL AND HISTOCHEMICAL STUDY OF DIGESTION IN THE BIVALVEARCTICA ISLANDICAL

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Abstract
The histology and enzyme histochemistry of the stomach and digestive gland of A. islandica were examined. Several enzymological and morphological features contribute to a well-developed system of extracellular digestion in this species. Extracellular exopeptidases and alkaline phosphatases are largely derived from primary and secondary ducts. The epithelium of these ducts also secretes esterases and acid phosphatases. Secretory activity of the ducts overshadows that of the stomach wall. Other digestive enzymes, including acid endopeptidases, are supplied by fragmenting digestive cells. Starvation has little apparent effect on most enzyme activity, but does depress secretion of alkaline phosphatases and intracellular production of acid phosphatases. These enzymes may be induced under appropriate feeding conditions. The primary ducts in A. islandica are organized into many ciliated folds, rather than separate rejection and acceptance tracts characteristic of anisomyarian bivalves. This structural arrangement insures that enzymes present in digestive cell fragments are utilized in the gastric cavity, rather than being rejected by the intestinal groove. Mature and immature basophilic cells, intermediate cells and mature and immature digestive cells are present in the digestive tubules. The 2 types of digestive cells are distinguishable on the basis of cellular inclusions, ability to phagocytize particulate matter, metachromatic staining, and nuclear structure. Cytological and histochemical evidence suggests that, in the digestive tubules of A. islandica, there is only 1 basic cell type, which passes through a number of distinct phases in its life history.

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