A class of fungi in the subdivision Mastigomycotina. They comprise a group of heterotropic, fungallike organisms that are classified with the zoosporic fungi (Mastigomycotina) but in reality are related to the heterokont algae. They are distinguished from other zoosporic fungi by the presence of biflagellate zoospores: the shorter, anteriorly directed flagellum contains two rows of tripartite hairs; the longer, posteriorly directed flagellum is naked and whiplash. Some taxa are nonzoosporic. The thallus may be unicellular, or mycelial and composed of filamentous hyphae that are coenocytic (without cross walls), at least in the early stages of development. The main component of the cell walls is cellulose, or cellulose and chitin. Asexual reproduction involves the release of zoospores from sporangia; in some taxa the sporangium germinates with outgrowth of a germ tube. Sexual reproduction occurs when an oogonial cell is fertilized by contact with an antheridium, resulting in one or more oospores. At least in some, possibly all, species, the thallus is diploid.