A comparison has been made between the staining reactions and histochemical properties of the non-calcareous material (conchiolin) from the different layers of the shells of Anodonta cygnea, Mytilas edulis, and Ostrea edulis. Acid hydrolysates of the conchiolin protein have been analysed qualitatively by paper chromatography.

The composition of the conchiolin in Anodonta confirms the view that corresponding layers of the valves and ligament represent modifications of the same layers of the shell. In this bivalve, the properties of the outer layers of the valves and ligament are closely comparable with each other, and also with those of the periostracum and the fusion layer of the ligament. All these regions consist of a quinone-tanned protein, hydrolysates of which are rich in phenolic amino-acids, especially tyrosine, and in glycine.

Much of the periostracal conchiolin in Mytilus shows basically the same properties as the periostracum in Anodonta. However, the outer layers of the valves and ligament in Ostrea and Mytilus each exhibit progressively greater specialization compared with the situation in Anodonta. This is most marked in Mytilus where these components differ completely in character.

The conchiolin in the inner shell layers differs markedly in composition from that in the outer layers in Anodonta and Ostrea and from the periostracum in Mytilus.Hydrolysates of its protein constituent contain appreciably more aspartic acid and glutamic acid but much smaller amounts of phenolic amino-acids. The protein is only lightly tanned. Although in these properties the corresponding inner layers of the valves and ligament appear fundamentally alike, each component has certain specialized features. It is suggested that the modifications shown by she protein of the inner ligament layer, which is characterized by a high content of proline and methionine, are correlated with the specialized function of this region of the shell.

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