Since multicellularity evolved independently in every major group of microorganisms, the blurred distinction between single-celled and many-celled organisms has become obsolete. The protists are divisible into about 35 unambiguous groups called phyla. They provide many examples of biological principles—including the prevalence of independent trends toward multicellularity. One illustration involves cellular slime molds. These heterotrophs undergo an extraordinary sequence of events during their life history. The story begins with single cells, indistinguishable from common amoebas. When starved, they begin to swarm. Soon they combine into a slimy mass of many nucleated amoeba cells called a pseudoplasmodium. The pseudoplasmodium in turn forms ...(100 of 16671 words)
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