Dr. Ritchie is to be congratulated for his paper on the biology and control of the oncomelanian snails. We, the Oriental people, are especially thankful for his own painstaking studies on this problem in Japan. In the Orient there are about 33 million persons suffering from schistosomiasis japonica. It is, therefore, one of the major medical problems in that part of the world.
His review of the subject is so complete that it is difficult to add anything to it. For some time, however, the distribution of the snail vectors has puzzled me. As you know, in Japan the distribution of O. nosophora is confined to only five small areas. When I visited these areas I was impressed by the fact that snails were further restricted to small, isolated habitats. For many years, although the Japanese have not succeeded in eliminating the vectors in an endemic area, the mollusks have not spread into adjacent areas.