Observations on the Mantle of the Lamellibranchia

Abstract

The variation in histology between the epithelial regions of the mantle in Anodonta cygnea concerned with the secretion of the different layers of the shell has been investigated. The epithelia on the outer surface of the outer mantle fold which secrete the outer layer of the ligament and the corresponding outer calcareous layer of the valves are basically alike in structure. They are also comparable in appearance with the epithelium on the inner surface of the outer mantle fold which secretes the periostracum and with that of the fused outer folds responsible for the formation of the fusion layer of the ligament. All these epithelial regions can readily be distinguished histologically from the epithelia of the general outer mantle surface and of the mantle isthmus which produce the inner layers of the valves and ligament respectively. The histological differentiation of the mantle epithelium in Anodonta is compared with that of Mytilus edulis and Ostrea edulis. The degree of zonation of the epithelium can be correlated with the extent to which the shell layers differ in composition from one another. It is suggested that the variation in histological appearance between epithelia from different areas of the mantle is partly determined by the relative amounts of non-calcareous material (conchiolin) which these epithelia produce and by the rate of their secretory activity. The different epithelial regions of the mantle in Anodonta and other lamellibranchs vary appreciably in their reactions to histochemical tests for alkaline phosphatase, ribonucleic acids, and glycogen. In Anodonta, the epithelial cells of the outer mantle fold differ markedly from those which secrete the inner layers of the shell in that they exhibit high alkaline phosphatase activity and appear to be rich in cytoplasmic ribonucleic acids. In contrast, the epithelium of the general outer surface of the mantle is distinguished by a high content of glycogen, a condition which also occurs in many other bivalves. It is interesting to note that alkaline phosphatase can be demonstrated in epithelia which secrete non-calcareous regions of the shell. In all the lamellibranchs investigated, ribonucleic acids can be detected in relatively higher concentration in epithelial cells concerned principally with the secretion of conchiolin.