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1.
An animal that does not have a backbone. Insects, arachnids, starfish, jellyfish, octopuses, sponges, and corals are all invertebrates.
Invertebrates are the most populated animals on the planet.
by David March 06, 2004
 
2.
(n). A spineless creature, lacking a backbone. Adapted for use in human society to describe someone who lacks character or resolve.
Have you ever met anyone who didn't have enough self control in an argument to keep his hands off his spouse? Invertebrate.
by DarkVic July 10, 2008