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an order of fungi of the subclass Oomy-cetidae of the class Phycomycetes. The fungi have a well-developed noncellular mycelium, whose slender hyphae embed themselves in nutrient substrate by means of rhizoids. They develop prolifically on the surface of the substrate, undergoing asexual and sexual reproduction. Asexual reproduction is effected by zoosporangia, and sexual reproduction by oogonia and antheridia.
In asexual reproduction pear-shaped zoospores with two flagella at the anterior end develop in the zoosporangia. Mature zoospores, upon emerging from the zoosporangia, float for some time in the water and then stop, covering themselves with a membrane and drawing their flagellae inside. They then shed the membrane and transform into another, usually budlike, stage, also having two lateral flagellae—one directed forward and the other backward. Upon settling on a suitable substrate, these secondary zoospores give rise to mycelium.
With sexual reproduction the contents of the antheridia are transfused through special processes into the oogonia, in which there are, as a rule, several egg cells. The fertilized egg cells—oospores—become clad in a two-layer membrane and, after a period of rest, they sprout, developing in the zoosporan-gium. The sprouting of an oospore is accompanied by reduction division of its diploid nucleus (seeMEIOSIS). Thus, the oospores are the only diploid phase of development of these fungi. The best-known genera are Saprolegnia and Achlya, which develop saprophytically on dead insects or parasitize roe and adult fishes, often causing mass illness or death to the hosts.
M. A. LITVINOV