Most classification systems define animals as being multicellular, thereby excluding protozoa, which are a diverse group of microscopic single-celled organisms. In most ways, protozoa do meet the general requirements to be considered animals, and some scientists classify them as single-celled animals.
Like all animals, protozoa have eukaryotic cells, meaning cells with a nucleus and membrane-bound organelles. They are, with a few exceptions, motile, either using cilia or flagellae to move about, or by simply oozing from one place to another as in the case of amoeba. Unlike plants, they cannot synthesize carbohydrates from smaller molecules and must absorb nutrition by seeking out and consuming other organisms. Being single-celled, protozoa lack the extracellular matrix of collagen and elastic glycoproteins that provides a flexible structure for the bodies of larger animals.