Unknown Algae

[TomB]

I have just found a what I believe to be an algae but could be a ciliate. It is ovoid slightly narrower at one end, about 200um long, greyish in colour, globular, cilia or flagella covering surface and rolls across the slide, it is filled at least twenty spherical shapes from 25 - 45 um in size. My first thought was it may be a Volvox but all pictures of them appear green and it isn't spherical. In forty years at looking at water not seen anything like it before, any ideas please?