An invertebrate is an animal that does not have a backbone. Ninety-seven percent of all animal species are invertebrates. Insects, Annelids, Mollusks, Echinoderms, Protozoa, Crustaceans, and Arachnids are all invertebrates.
Invertebrates share four common traits:
1. They do not have a backbone.
2. They are multi-cellular. All the cells have different responsibilities in keeping the animal alive.
3. They have no cell walls, like all other animals.
4. They reproduce by two reproductive cells, or gametes, coming together to produce a new organism of their species.