From Academic Kids
Invertebrate is a term coined by Jean-Baptiste Lamarck to describe any animal without a spinal column. It therefore includes all animals except vertebrates (fish, reptiles, amphibians, birds and mammals).
Since invertebrates include all animals except a certain group, invertebrates form a paraphyletic group, but, despite not forming a "natural group" (that is, monophyletic), "invertebrate" is still a widely used term. It is not uncommon for books entitled "Invertebrate Zoology" to be found. This reflects the bias in society and also in zoology towards larger, more complex animals that are more closely related to humans. Thus, there are relatively many scientists studying (and relatively much funding available for the study of) birds, mammals, reptiles, and so on, but far fewer scientists studying invertebrates, even though invertebrates include 97% of all animal species.