The cryptomonads (Cryptophyta, Cryptomonadida) are a group of tiny flagellates including mostly algal forms. The cells are around 10-50 μm in size and flattened in shape, with an anterior groove or pocket containg refractile extrusomes called ejectisomes. There are typically two slightly unequal flagella inserting nearly parallel next to the pocket. The mitochondrion has flat cristae.
Most cryptomonads contain a chloroplast, although a few are colorless, either containing a leukoplast (e.g. Chilomonas) or lacking plastids entirely (e.g. Goniomonas). Instead of being contained directly, there is a reduced eukaryote symbiotic (the highly reduced remains of a red alga) within the cell, which contains a normal prokaryote chloroplast. In total then, the plastid is bound by four membranes, with a tiny nucleus (nucleomorph) between the middle two. It typically has chlorophylls a and c, much like the dinoflagellates (with which the cryptomonads were formally grouped) and heterokonts. The nucleomorph formerly was the nucleus of the red alga; the periplastidial space between the middle two surrounding membranes, which is filled with eukaryotic ribosomes and starch grains, corresponds to the former cytoplasm of the red alga. Since the plastid of the cryptomonads is derived from a red alga, it is also termed a rhodoplast to differentiate it from the chloroplasts of the green lineage.
A few cryptomonads can form a palmelloid stage of organization, but turn easily to free-living flagellates by escaping from the mucus. Cryptomonads occur in freshwater habitats as well as in marine or brackish waters. Well-known examples include Cryptomonas and Chilomonas.