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).
Lamarck divided these animals into two groups, the Insecta and the Vermes, but now, they are classified into over 30 phyla, from simple organisms such as sponges and flatworms to complex animals such as arthropods and mollusks.
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. Invertebrates include 97% of all animal species.
For a full list of animals considered to be invertebrates, see animal. All the listed phyla are invertebrates along with two of the three subphyla in Phylum Chordata: Urochordata and Cephalochordata. These two, plus all the other known invertebrates, have only one cluster of Hox genes, while the vertebrates have duplicated their original cluster more than once.
Phylums and common examples
Insects are invertebrates and are taxonomically referred to as the class Insecta. They are the most numerous and most widelyspread terrestrial taxon within the phylum Arthropoda. Insects are the most diverse group of animals on the earth, with around 925,000 species described—more than all other animal groups combined: "Indeed, in no one of her works has Nature more fully displayed her exhaustless ingenuity," Pliny the Elder exclaimed. Insects may be found in nearly all environments on the planet, although only a small number of species have adapted to life in the oceans where crustaceans tend to predominate.
There are approximately 5,000 dragonfly species, 2,000 praying mantis, 20,000 grasshopper, 170,000 butterfly and moth, 120,000 fly, 82,000 true bug, 350,000 beetle, and 110,000 bee and ant species described to date. Estimates of the total number of current species, including those not yet known to science, range from two to thirty million, with most authorities favoring a figure midway between these extremes.
The study of insects is called entomology.