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Water moulds (or water molds: see spelling differences) also known as Oomycetes are a group of filamentous, unicellular protists, physically resembling fungi. They are microscopic, absorptive organisms that reproduce both sexually and asexually and are composed of mycelia, or a tube-like vegetative body (all of an organism's mycelia are called its thallus). The name "water mould" refers to their earlier classification as fungi, which stemmed from their preference for conditions of high humidity and running surface water, although they are now known to have evolved separately and show a number of differences. For instance, their cell walls are composed of cellulose rather than chitin and generally do not have septations. Also, in the vegetative state they have diploid nuclei, whereas fungi have haploid nuclei.