(me-sew-din-ee-um) common marine ciliate, small with a wreath which includes forward and rearward pointing cilia. Often small tentacles emerge from the apex of the cell. The tentacles may seem forked at their tips They have a jumping motion often coming to rest, spinning for a few seconds before dashing off to somewhere else. One species includes red algal symbionts and may occur in sufficient numbers to cause a red tide - but not toxic. Phase contrast.