Is this Volvatella ayakii ?

Dear Nishina,
I think this is probably Hamatani's Volvatella ayakii. He said the main distinguishing features were the shape of the shell, the external appearance of the living animal and the radular morphology. Your animals are white, and have a constriction in the shell, as described by Hamatani. However most species of Volvatella were described from the shell alone, which is a pretty unreliable character when the shell is only feebly calcified and often becomes mishapen when dried. My feeling is that we might as well call it Volvatella ayakii until the other, and much earlier, names are sorted out.

It would be interesting to know the name of the Caulerpa you found this species on. Hamatani's specimens were found on Caulerpa brachypus.
Best wishes,
Bill Rudman