**Acknowledgement**

This work was supported by a Grant-in-Aid for Scientific Research (C) (2) (No.11660205), (C)(No.18580204), (C)(No.20580220) and (B)(No.20380110) from Japan Society for the Promotion of Science. We are grateful to Dr.Hiroshi Kajihara (Hokkaido University) for his helpful advice on ribbon worms.
