A NOTE by T. M. Yanagita and T. Wada1 suggesting, after C. H. Brown2, the possible keratin nature of coelenterate nematocysts has directed my attention to the subject. In 1952 I had observed the pronounced metachromatic colour produced by dilute thionin on large nematocysts from a siphonophore, Halistemma sp. It is known that the metachromatic colour of a dye, as taken up by a substance, is characteristic of an acidic mucopolysaccharide content, associated or not, with proteins to form mucoproteins3. The fact interested me, since I had studied such substances in the envelopes of pagurid and cephalopod spermatophores4.