The image to the right represents a zoosporangium found in some chytrids (See links page). The predominantly aquatic chytrids have largely been overlooked by biologists. The group consists of around 100 genera with approximately 1000 species. They are the only members of the kingdom fungi that produce motile cells at some period in their life. The motile cells of these organisms, the zoospores and gametes, each possess a single, posteriorly directed, whiplash flagellum. The flagellum, being entirely typical for a primitive eukaryote, is almost certainly a feature retained from ancestral protists. Since a structure as complex as the flagellum will not have arisen more than once, this indicates an aquatic ancestry for the chytrids extending at least to the earliest flagellate eukaryotes. The occurrence of zoospores in one class of true fungi suggests an aquatic ancestor for all true fungi, even though most are terrestrial. Using 18S ribosomal RNA gene sequence data, it is estimated that terrestrial fungi diverged from chytrids approximately 550 - 600 millions of years ago, coincidently around the time with the origin of land plants. It is thus postulated that the move to land for plants was facilitated by some forms of symbiotic fungi, and vice-versa. 

Generally, the chytridiomycota are typically unicellular, or primitive chains of cells, and attached to a food substrate by tapering rhizoids. Sexual reproduction is by the fusion of male and female motile gametes, the product being a resting spore. Asexual reproduction is by cytoplasmic cleavage in a sporangium, producing motile zoospores. Many chytrids are saprobes, however some are pathogens of plants, animals and even fungi. The chytridiomycota are divided into the orders Spizellomycetales, Chytridiales, Blastocladiales, Monoblepharidales and Neocallimasticales, all orders being recognised under an approach emphasising zoospore ultrastructure. Much study has gone into the so-called 'gut' fungi. Caecomyces and Piromyces are example genera found in stomachs of mammals. The divergence of the stomach chytrids from free-living chytrids probably occured after the evolution of their hosts (150-200 millions of years ago) but before the split of marsupial and placental mammals (100 millions of years ago), since in modern day, gut chytrids can inhabit both.