"having naturally no backbone," 1819, from Latin in- "not" (see in- (1)) + vertebratus (Pliny), from vertebra "joint or articulation of the body, joint of the spine" (see vertebra). As a noun, "an invertebrate animal," 1826.
word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."
In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
"bone of the spine," early 15c., from Latin vertebra "joint or articulation of the body, joint of the spine" (plural vertebræ), perhaps from vertere "to turn" (from PIE root *wer- (2) "to turn, bend") + instrumental suffix -bra. The notion would be the spine as the "hinge" of the body.
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<a href="https://www.etymonline.com/word/invertebrate">Etymology of invertebrate by etymonline</a>
Harper, D. (n.d.). Etymology of invertebrate. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/invertebrate