Microscopic fungi were the earliest to evolve. Although fungi are ostensibly opisthokonts: organisms with flagellate cells, only the earliest – chytrids – still flail flagella.
Chytrids are a primordial fungus. There are 1,000 species in 127 genera. Most are saprovores. Some are plant pathogens. All are aquatic. Many are freshwater dwellers.
Chytrids may reproduce asexually or sexually. Some do both. Regardless, the next generation starts out with a tail.
Chytrids spawn sport a flagellum, whether gametes or zoospores (motile asexual spores). Chytrids are the only true fungi that reproduce by zoospores.
Chytrids keep it simple. The thallus (body) of a mature chytrid commonly comprises just a zoosporangium, typically spherical, and a rhizoid system. The zoosporangium is the main body and producer of zoospores. The rhizoids absorb and digest food, as well as anchoring the chytrid to its chosen substrate.