Histochemical studies of the periostracum of the bivalve Meretrix casta during the early and late growth stages were carried out.

The periostracum is composed of an outer part formed of lipoproteins, the middle layer showing the presence of a sulphur containing protein and the inner-most similar to that of the outer layer.

The periostracum of M. casta shows in the early stages of growth, minute transverse canals traversing its entire width. In later stages, the canals appear to enlarge into large vesicles in the middle layer. But the canals are occluded by material which is indistinguishable from the surrounding region. The canals are exposed after treatment with detanning agents like alkaline stannite solution suggesting that the material in them may be tanned protein.

Changes taking place in the chemical composition of the periostracum during growth involve the occurrence of quinone tanning in the outer-most layer.

The middle layer of the periostracum which undergoes hardening by sulphur bonding in the later growth stages recall strongly the -S-S- bonding reported in some arthropod cuticle.