The species Ikedosoma pirotansis is significantly different fom the two other species of the genus Ikedosoma in having the dermal longitudinal muscles not aggregated into bundles, and terminal gonostome. On the basis of the disposition of the muscle layers of the body wall the secies is related to Thalassema, Anelassorhynchus, and Arhynchite, besides, it shares with Arhynchite a long presiphonal intestine. I. pirotansis has very little in common with Ochetostoma and other related genera. It, however, shows certain features in common with bonellids, namely, ventral accessory vessel as in Protobonellia, branched ad stalked anal vesicles as in Maxmulleria and Prometor, and terminal gonostome as in Eubonellia and Bonelliopsis. In many respects the species is nearer to Ikeda, but the latter has dermal longitudinal muscles outer to both circular and inner oblique layers which is contrary to the condition in I. pirotansis. Besides, the gonoducts are excessively numerous in Ikeda. Apparently the inclusion of I. pirotansis under Ikedosoma or Ikeda can hardly be justified, and a new genus Rubricelatus has been erected, the type species being Ikedosoma pirotansis Menon and DattaGupta. Dawydoff's (1959) arrangement with regard to the genera Echiurus, Urechis, and Ikeda seems appropriate. The family Ikedaidae Dawydoff can accomodate Ikeda and Rubricelatus n. gen. Thalassematidae of Bock can be emended to include the genera Thalssema, Anelassorhynchus, and Arhynchite on the one han, and Ochetostoma, Listriolobus, Ikedosoma, Lissomyema, and Platylobostoma on the other under two subfamilies, Thalassematinae Bock emended and Ochetostomatinae novo, separated on the basis of dermal longitudinal muscle bands. Rearrangement of the Bonellidan genera can be more realistic on the basis of the modification of gonoducts.
