Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a spine or backbone), which evolved from the notochord.

Well-known phyla of invertebrates include arthropods, mollusks, annelids, echinoderms, flatworms, cnidarians, and sponges.

The most familiar invertebrates include the Protozoa, Porifera, Coelenterata, Platyhelminthes, Nematoda, Annelida, Echinodermata, Mollusca and Arthropoda.

The body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, or spherical.

Sessile animals such as sponges are asymmetrical[13] alongside coral colonies (with the exception of the individual polyps that exhibit radial symmetry); Alpheidae claws that lack pincers; and some copepods, polyopisthocotyleans, and monogeneans which parasitize by attachment or residency within the gill chamber of their fish hosts).

Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH.

[19] Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodes and arthropods.

The tracheae are invaginations of the cuticular exoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometres up to 0.8 mm.

The smallest tubes, tracheoles, penetrate cells and serve as sites of diffusion for water, oxygen, and carbon dioxide.

In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a gill, or function essentially as normal, via a plastron.

They are connected by a thin bridge of exoskeleton and they function like a tiny pair of eardrums, but, because they are linked, they provide acute directional information.

Depending on where the song of the cricket is coming from, the fly's hearing organs will reverberate at slightly different frequencies.

Extensive research with model invertebrate species such as Drosophila melanogaster and Caenorhabditis elegans has contributed much to our understanding of meiosis and reproduction.

[30] In one extreme example, it is estimated that 10% of orbatid mite species have persisted without sexual reproduction and have reproduced asexually for more than 400 million years.

[30] Social behavior is widespread in invertebrates, including cockroaches, termites, aphids, thrips, ants, bees, Passalidae, Acari, spiders, and more.

[39] The Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.

[44] Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share some traits with them, excluding the hardened exoskeleton.

Invertebrates can be classified into several main categories, some of which are taxonomically obsolescent or debatable, but still used as terms of convenience.

[57] Some paleontologists suggest that animals appeared much earlier, possibly as early as 1 billion years ago[58] though they probably became multicellular in the Tonian.

Trace fossils such as tracks and burrows found in the late Neoproterozoic Era indicate the presence of triploblastic worms, roughly as large (about 5 mm wide) and complex as earthworms.

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 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 sea sponges and flatworms to complex animals such as arthropods and molluscs.

Although goal-directed evolution has been abandoned, the distinction of invertebrates and vertebrates persists to this day, even though the grouping has been noted to be "hardly natural or even very sharp."

Another reason cited for this continued distinction is that Lamarck created a precedent through his classifications which is now difficult to escape from.

The book also points out that the group lumps a vast number of species together, so that no one characteristic describes all invertebrates.

[64] The study of invertebrates has also benefited law enforcement, as arthropods, and especially insects, were discovered to be a source of information for forensic investigators.

[44] Two of the most commonly studied model organisms nowadays are invertebrates: the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans.

Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1,500 ancestral genes unique to animals.

[65] Invertebrates are also used by scientists in the field of aquatic biomonitoring to evaluate the effects of water pollution and climate change.