Class of algae which includes no single-celled species; almost all are marine, growing mostly in the intertidal regions. They are the dominant seaweeds in the colder waters of the northern hemisphere. They are typically olive brown or greenish in colour (at least when wet) due to the presence of the pigment fucoxanthin in the chloroplasts. Some fossil plants resembling Fucales (an order of brown seaweeds) have been reported from the Palaeozoic, but their fossil record is equivocal owing to their lack of calcification.