type of microscopic protozoa, 1855, from Modern Latin Amoeba, genus name (1841 in English, said to have been used 1830 by German naturalist Christian Ehrenberg), from Greek amoibe "change, alteration; exchange," which is related to ameibein "to change, exchange," from PIE *(e)meigw-, which is an extended form of root *mei- (1) "to change, go, move" or perhaps a separate root. So called for its constantly changing shape. An earlier popular name for it was proteus (1660s). Related: Amoebic; amoebiform; amoeboid.
occasional plural suffix of words ending in -a (see a- (1)), most of which, in English, are from Latin nominative fem. singular nouns (or Greek ones brought up through Latin), which in Latin form their plurals in -ae. But plurals in native -s were established early in English for many of them (such as idea, arena) and many have crossed over since. Now it is not possible to insist on purity one way or the other without breeding monsters.