Favorinus GRAY, 1850 is a splendid genus, the distinctive characters of which were introduced in parts by ALDER & HANCOCK, 1855, and somewhat more effectively by ODHNER, 1939 (see also MACNAE, 1954a, pp. 17-19). Eolis alba A. & H., 1844, Atlantic, forms the type of the genus. A series of species were added to this genus by the senior author (F. pacificus BABA, 1937; F. japonicus BABA, 1949; F. perfoliatus BABA, 1949; and F. mirabilis BABA, 1955), and it was revealed that the rhinophores in Favorinus are either simple, or bulbed, or even perfoliated according to different species. Here F. japonicus BABA was taken up for special study in anatomy, with the hope to understand the various genetic features more profoundly than before. Apparently this species is one of those nearest to the type of the genus, particularly in having bulbs on the mid-length of the rhinophores.