The word "invertebrate" comes from the Latin word vertebra, which means a joint in general, and sometimes specifically a joint from the spinal column of a vertebrate. The jointed aspect of vertebra is derived from the concept of turning, expressed in the root verto or vorto, to turn.[5] The prefix in- means "not" or "without".

A group of animals that do not have a backbone. 1-2 million species of animals. Taxonomic rank of many invertebrate groups is not established. They inhabit all habitats: water, earth surface, soil, air. Some, being parasites, live in other organisms. Invertebrates play the most important role in the Earth's cycle of substances and energy. Invertebrates include the Protozoa, Sponges, Coelenterates, Inferior Worms, Molluscs, Sphenomena, Echinoderms and some other types.

Invertebrates vary widely in size, from the 50 µm rotifers to the colossal 9-10 m squid. With a few exceptions, such as Porifera, invertebrates usually have a body made up of differentiated tissues. There is also usually a digestive chamber with one or two openings to the outside. The nerve cells of invertebrates are different from those of mammals. Invertebrate cells fire in response to the same stimuli as mammals, such as tissue injury, high temperature or pH changes. The first invertebrate in which a nerve cell was discovered was the medicinal leech Hirudo medicinalis. Nociceptor-mediated learning and memory have been described in the sea hare Aplysia. Clam neurons are able to detect increased pressure and tissue injury. Neurons have been found in a number of invertebrate species, including annelids, molluscs, nematodes and arthropods.

The term 'invertebrates' covers several phyla. Sponges (Porifera) are one of them. They were long thought to have diverged from other animals at an early stage. They lack the complex organisation characteristic of most other phyla. Their cells are differentiated, but in most cases not organised into separate tissues. Sponges usually feed by taking in water through their pores. Some suggest that sponges are not so primitive, but may be secondary simplifications. Ctenophores and cnidarians, which include sea anemones, corals and jellyfish, are radially symmetrical and have digestive chambers with a single opening that serves as both mouth and anus. Both have distinct tissues, but they are not organised into organs. There are only two main germ layers, ectoderm and endoderm, with scattered cells in between. This is why they are sometimes called diploblasts. Echinoderms are radially symmetric and exclusively marine, and include sea stars (Asteroidea), sea urchins (Echinoidea), brittle stars (Ophiuroidea), sea cucumbers (Holothuroidea), and feather stars (Crinoidea). Invertebrates also include the largest animal phylum, Arthropoda, which includes crabs and other crustaceans. It also includes the two most successful animal phyla, the molluscs and the annelids. The first, which is the second largest animal phylum in terms of the number of species described, includes animals such as snails, clams and squid, and the second includes segmented worms such as earthworms and leeches. The two groups were long thought to be closely related because of the common presence of trochophoric larvae, but annelids were thought to be closer to arthropods because they are both segmented. This is now generally regarded as convergent evolution, due to the many morphological and genetic differences between the two phyla.

Invertebrates can be divided into several main categories, some of which are taxonomically obsolete or controversial, but are still used as convenient terms.

Sponges (Porifera)

Comb jellies (Ctenophora)

Medusozoans and corals (Cnidaria)

Acoels (Xenacoelomorpha)

Flatworms (Platyhelminthes)

Bristleworms, earthworms and leeches (Annelida)

Insects, springtails, crustaceans, myriapods, chelicerates (Arthropoda)

Chitons, snails, slugs, bivalves, tusk shells, cephalopods (Mollusca)

Roundworms or threadworms (Nematoda)

Rotifers (Rotifera)

Tardigrades (Tardigrada)

Scalidophores (Scalidophora)

Lophophorates (Lophophorata)

Velvet worms (Onychophora)

Arrow worms (Chaetognatha)

Gordian worms or horsehair worms (Nematomorpha)

Ribbon worms (Nemertea)

Placozoa

Loricifera

Starfishes, sea urchins, sea cucumbers, sea lilies and brittle stars (Echinodermata)

Acorn worms, cephalodiscids and graptolites (Hemichordata)

Lancelets (Amphioxiformes)

Salps, pyrosomes, doliolids, larvaceans and sea squirts (Tunicata)