Cryothecomonas aestivalis

Colourless flagellates; in the free, motile stage, oblong to oval, 9-12 microns long and 4-5 microns wide; two apically inserted flagella, anteriorly directed flagellum 15 microns long; posteriorly directed flagellum up to 25 microns. Feeds on the marine planktonic diatom Guinardia delicatula. Flagellate penetrates the diatom frustule. Trophonts gradually phagocytize the host cytoplasm by means of a pseudopodium, which emerges posteriorly through a gap in the theca. Theca delicate, consisting of two layers, occasionally lacking. Trophonts and division stages with shortened, in part basally thickened flagella. Mature trophonts give rise to 8-32 new flagellates (swarmers). Defecation before the last division.