A small class of nonmotile, photosynthetic, unicellular algae, segregated from the class Xanthophyceae (Tribophyceae) on the basis of cytological, ultrastructural, and biochemical features of the vegetative cell and zoospore. Because of their lack of chlorophyll b, these organisms may be placed in the division Chromophycota, but their lack of chlorophyll c, in contrast to other chromophytes, supports recognition at a higher level (such as the division Eustigmatophyta). Only a dozen species in three genera are known. These live chiefly in fresh water, but also in marine habitats and in soil.