When first secreted the radula consists of chitin and protein. In the bases and cusps of the teeth the reactions for tyrosine, tryptophane, and chitin become negative, and are replaced by reactions for amino groups and argentaffin material. The bases of the teeth become impregnated with iron. Subsequently the reactions for amino groups and argentaffin material become negative in the cusps and they then become impregnated with iron and silica. Very few changes have been observed in the radular membrane.
It is suggested that the occurrence of quinone tanning might account for these histochemical changes observed in the radula, but phenols derived from tryptophane would appear to be more important in the tanning of the radula than those derived from tyrosine.