any of about 500 species of primitive organisms containing true nuclei and resembling both protozoan protists and fungi.

The term slime mold embraces a heterogeneous assemblage of organisms whose juxtaposition reflects a historic confusion between superficial resemblances and actual relationships. The Myxomycetes (q.v.; true slime molds) are characterized by a plasmodial stage and definite fruiting bodies. Other slime molds include Protostelia (minute, simple slime molds), Acrasia (Acrasieae) (cellular slime molds), Plasmodiophorina (Plasmodiophoromycota) (parasitic slime molds), and Labyrinthulina (net slime molds). Slime molds are found worldwide and typically thrive in dark, cool, moist conditions such as prevail on forest floors. Bacteria, yeast, molds, and fungi provide the main source of slime-mold nutrition, although the Plasmodiophorina feed parasitically on the roots of cabbage and other mustard-family plants.

The life cycle of the Myxomycetes is, allowing for minor variations, representative of that of slime molds generally. The cycle begins with a spore that has a diameter of 4 to 15 micrometres (1 micrometre equals 0.001 mm, or 0.000039 inch) and that, in the presence of water, releases a small mass of cytoplasm called a swarm cell. It is propelled by whiplike appendages (flagella) until it comes in contact with a surface and puts forth pseudopods (lobes of cellular material) that allow it to creep along. In its creeping phase it resembles an amoeba and is known as a myxamoeba. Both swarm cells and myxamoebae function as sex cells (gametes), and the fusion of two such cells constitutes the reproductive act of myxomycetes that begins the next stage of growth, the plasmodium.

As the flagella are permanently retracted, the fertilized cell begins to grow by repeated division of its nuclei. The plasmodium moves gradually in successive waves, creating a characteristic fan shape. A layer of slime, in some species similar to saliva or mucus, covers the whole plasmodium.

The most remarkable metamorphosis of the slime molds occurs next: the growth from the shapeless plasmodium of an intricately organized spore case, or sporangium. Droplets form at the cell wall and coalesce to form a cushion and then a stalk that can grow to be 1.25 cm (0.5 inch) wide and 2.5 cm (1 inch) tall. As the column changes to purple and then black, the sporangium forms at its tip, filled with the dark spores. The sporangium wall dries and disintegrates, allowing air currents or a sudden movement to release the spores and begin the cycle again.