There are two main slime mold phyla in Kingdom Protista, each with its own distinct characteristics.
Phylum Acrasiomycota (cellular slime molds)
Cellular slime molds closely resemble amoebas in structure. These molds live independently until food runs out. A starving amoeba secretes the hormone cyclic AMP into the environment. Other amoebas detect the cyclic AMP as a food source and aggregate from great distances to follow the concentration gradient to the dying amoeba. The amoebas then attach to one another and become what seems to be a functioning multicellular organism. The moving slug finds a suitable habitat before forming itself into a diploid fruiting body called a sorocarp, which releases encysted amoebas or diploid macrocysts. The released amoebas live independently until food resources are depleted, then the cycle is repeated.
Phylum Myxomycota (plasmodial slime molds)
A plasmodial slime mold is essentially one large cell, which is possible due to the mold's multiple nuclei that create many RNA. This type of mold lives as a non-walled multi-nucleated mass of slime in its feeding phase. The nuclei of the mold, which can be either a diploid or haploid species, undergo mitosis. Fruiting bodies are formed under harsh conditions. The spores produced by the sporangium are either diploid or haploid, depending on the ploidy of the plasmodium as a whole. The spores undergo meiosis to produce gametes. After the spores are released, the gametes fuse when they come in contact with one another and undergo repeated mitosis to form a multinucleate plasmodium.
The molds of these phylums are similar in that those of phylum Acrasiomycota converge to form a seemingly individual organism from many amoebas, while a mold of the phylum Myxomycota is essentially one large cell that functions as individual cells because the large cell is multi-nucleated. The reproductive cycles of the two phylum differ in that cellular slime mold gives rise to fruiting bodies and release amoebas, while plasmodial slime molds produce gametes that must fuse to and undergo mitosis to form the new plasmodial slime mold.