Cells with four lateral arm-like extensions of equal length; posterior tail; cell wall delicate, only basally and in the lateral extensions separated from the protoplast; flagella two of equal length; basal apparatus with two additional dormant basal bodies, and two system II fibres which branch and extend to mitochondrial profiles; chloroplast cup-shaped; pyrenoid present or absent; eyespot present; contractile vacuoles two, anteriorly located; asexual reproduction by 2-8 zoospores; sexual reproduction isogamous, gametes simpler than vegetative cells; zygotes with thick, smooth cell wall; akinetes and palmelloid stages known; marine and brackish genus, though species tolerate wide range of salinities and may occur in freshwater pools near the sea, occasionally also in much-polluted inland freshwater habitats.