Rhodophytes are just about the only eukaryotes to possess phycobilins, a class of water-soluble pigments; the only other eukaryotes to have them are the Cryptomonads. This and their lack of accessory chlorophylls, such as those found in most other photosynthetic eukaryotes, suggest that the plastids of rhodophytes were gained independently from these other groups. Rhodophytes also lack the eukaryotic flagella, or undulipodia which characterize most other eukaryotes. This has led some experts (e.g. Tappan, 1980) to regard red algae as extremely primitive eukaryotes, a position which is sometimes challenged today.
Classification within the Rhodophyta is rather unstable, and is currently undergoing much investigation and revision. For more information see:
Also visit Derek Keats' page on the Families and Subfamilies of nongeniculate Corallinales, including a host of generic descriptions.