CoachAn organism is unicellular when a single cell carries out every function of life — nutrition, exchange, movement, reproduction — and lives on its own. It is colonial when cells of one kind stay attached after dividing but each remains able to live alone if separated. It is multicellular when its cells are of several types, depend on one another, and only a few of them (the germ line) leave descendants. Unicellular organisms include all archaea, nearly all bacteria, and most lineages of eukaryotes; the informal name protist covers the unicellular eukaryotes, which are not one group but many.
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Example 1.2 (Six inhabitants of a drop)
Escherichia coli, a rod long, swims with a tuft of flagella and divides every twenty minutes when fed. Chlamydomonas, a green alga across, rows with two flagella toward the light it detects with an eyespot. Paramecium, a ciliate of , is covered with thousands of cilia, feeds through a gullet and bails water out with two contractile vacuoles. A diatom lives in a two-part shell of silica, ornamented with pores. Amoeba flows by extending pseudopods and engulfs what it meets. A yeast cell buds a daughter from its side. Six answers, of widely different size and machinery, to one problem.