Endosymbiosis - organisms living together, one inside the other. Slime molds live in damp soil and the moist, decaying bark of rotting trees. Organelles Contractile vacuole - a membrane enclosed vesicle that removes excess water. Food vacuole - a membrane enclosed vesicle where engulfed food is digested. A few protists are surrounded by a plasma membrane.
Protozoans have internal shells that they produce.
Many algae and other protists have cell walls. Although most protists reproduce asexually and sexually, some groups are asexual only. Sexual reproduction is syngamy - the union of two gametes. For convenience, we will continue to use the term protist in an informal sense to refer to unicellular eukaryotes and their close multicellular relatives. Archaezoa lack mitochondria and may represent early eukaryotic lineages.