Invertebrate is a term that describes any animal without a spinal column. The group includes 97% of all animal species — all animals except those in the Chordate subphylum Vertebrata (fish, reptiles, amphibians, birds and mammals).
Carolus Linnaeus' Systema Naturae divided these animals into only two groups, the Insecta and the Vermes. Jean-Baptiste Lamarck, who was appointed to the position of "Curator of Insecta and Vermes" at the Muséum National d'Histoire Naturelle in 1793, both coined the term "invertebrate" to describe such animals and divided the original two groups into ten, by splitting off Arachnida and Crustacea from the Linnean Insecta, and Mollusca, Annelida, Cirripedia, Radiata, Coelenterata and Infusoria from the Linnean Vermes. They are now classified into over 30 phyla, from simple organisms such as sponges and flatworms to complex animals such as arthropods and molluscs.
Since invertebrates include all animals except a certain group, invertebrates form a paraphyletic group. (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.
Phyla and common examples
- Porifera — sponges
- Cnidarians — jellyfish
- Platyhelminthes — flatworms
- Nematoda — roundworms
- Annelida — segmented worms
- Echinodermata — sea stars, sea urchins, sea cucumbers
- Mollusca — squid, snails
- Arthropoda — insects, arachnids, crustaceans
- Bryozoa — sea mats (occasionally resemble corals)
- A. R. Maggenti & S. Gardner (2005). Online Dictionary of Invertebrate Zoology.