Invertebrate is a term coined by Jean-Baptiste Lamarck to describe any animal without a spinal column. It therefore includes all animals except those in the subphylum vertebrata (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.
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.
- Annelids - The annelids, collectively called Annelida (from Latin annellus "little ring"), are a large phylum of animals, comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches.
- Arthropods - Arthropods (phylum Arthropoda) (from Greek meaning foot) are the largest phylum of animals and include the insects, arachnids, crustaceans, and others.
- Chordata - Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates.
- Cnidarians - Cnidaria (silent c - "nettle") is a phylum containing some 11000 species of relatively simple animals found exclusively in aquatic, mostly marine, environments.
- Echinoderms - Echinoderms (Phylum Echinodermata, from the Greek for spiny skin) are a phylum of marine animals found at all depths. This phylum appeared in the early Cambrian Period and contains about 7,000 living species and 13,000 extinct ones.
- Molluscs - The mollusks or molluscs are the large and diverse phylum Mollusca, which includes a variety of familiar animals well-known for their decorative shells or as seafood.
- Nematodes - The nematodes or roundworms (Phylum Nematoda from Gr. nema, nematos "thread" + ode "like") are one of the most common phyla of animals, with over 20,000 different described species (over 15,000 are parasitic).
- Sponges - The sponges or poriferans (from the Greek poros "pore" and ferro "to bear") are animals of the phylum Porifera.
- Trematodes - The Trematoda is a class within the phylum Platyhelminthes, which contains two groups of parasitic worms.