The histochemical characteristics and ultrastructure of the mantle of Gomphina veneriformis, were described using light and electron microscopy. When the mantle was distinguished into marginal, middle, and apical zones, the marginal mantle was also divided into 4-folds (inner-inner, inner-outer, middle, and outer fold), making it similar to the typical marginal mantle of Veneridae. The mantle thickness displayed a trend of reduction from the marginal zone to the apical zone. However, epidermal thickness was the thickest in the middle zone, with a thicker epidermal layer in the outer epidermis than the inner epidermis. The inner epidermis of the transitional area between the marginal and middle mantle is a simple columnar layer with well-developed microvilli on the free surface. Although the outer epidermis is also a simple columnar layer, few microvilli are present. The inner epidermis of the middle and apical zone is simple cuboidal layer, whereas the outer epidermis is a simple columnar layer. Ciliated cells are columnar in shape, with development of microvilli and cilia on the free surface. In the apical cytoplasm, numerous mitochondria were observed along with granules of low electron density. The distribution of mucous cells was found to be higher in the marginal mantle than those in the apical mantle. The proportion of mucous cells in the inner and outer epidermis in the marginal zone was 20.3% and 2.8%, respectively, with a high ratio in the inner epidermis. The result of this study illustrates that there are three types (A, B, and C) of secretory cells in the mantle, all of which are unicellular glands. Furthermore, in type A cells, the most ubiquitous of the three types and a typical goblet cell, the mucous substance contained in these cells was a weakly acidic or neutral carboxylated mucopolysaccaride. A periostracal groove was located between the middle fold and the outer fold, where the periostracum originates. Intermediate cells and basal cells were observed in the transitional area between the middle fold and the outer fold. Intermediate cells are elliptical with numerous vacuoles of various sizes. Basal cells have microvilli on the free surface, and there are membrane-bound granules between the microvilli and immature periostracum. The periostracum, as it moves further from the periostracal groove, displays higher electron density and gets thicker. Mature periostracum is composed of a homogeneous layer and a fibrous layer. The homogeneous layer appeared darker than the fibrous layer because of higher electron density.
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Vol. 26 • No. 2