Invertebrate

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Invertebrate is a term coined by Jean-Baptiste Lamarck to describe any animal without a backbone or spinal column. The group includes about 97% of all animal species; that is, all animals except vertebrates, (subphylum Vertebrata of the phylum Chordata), which have a backbone or spinal column. Vertebrates include fish, reptiles, amphibians, birds and mammals.

Lamarck divided these animals into two groups, the Insecta and the Vermes, but they are now classified into about 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. All the phyla of animals 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.

Select phyla and common examples

Porifera: sponges

The sponges or poriferans are animals of the phylum Porifera. They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter. With no true tissues (parazoa), they lack muscles, nerves, and internal organs. There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m (29,000 feet) or further. The fossil record of sponges dates back to the Precambrian era.

Cnidarians: jellyfish, corals, sea anemones

Cnidaria (silent c - pronounced /naɪˡdeɹiə/ from New Latin cnida, fr. Gk κνιδη "nettle", "sea anemone"[1]) is a phylum containing some 11,000 species of relatively simple animals found exclusively in aquatic, mostly marine, environments. Cnidarians get their name from cnidocytes, which are specialized cells that carry stinging organelles. The corals, which are important reef-builders, belong here, as do the familiar sea anemones, jellyfish, sea pens, sea pansies and sea wasps. The names Coelenterata and Coelentera were formerly applied to the group, but as those names included the Ctenophores (comb jellies), they have been abandoned. Cnidarians are highly evident in the fossil records, having first appeared in the Precambrian era.

Platyhelminthes: flatworms

The flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of relatively simple soft-bodied invertebrate animals. With about 25,000 known species they are the largest phylum of acoelomates. Flatworms are found in marine, freshwater, and even damp terrestrial environments. Most are free-living forms, but many are parasitic on other animals. There are four classes: Trematoda (Flukes), Cestoda (Tapeworms), Monogenea, and Turbellaria.

Nematoda: roundworms

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). They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants and animals, humans included. Only the Arthropoda are more diverse.

Annelida: earthworms

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. They are found in most wet environments, and include many terrestrial, freshwater, and especially marine species (such as the polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimeter to over 3 metres (the seep tube worm Lamellibrachia luymesi).

Echinodermata — sea star, sea urchins, sea cucumbers

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. Five or six classes (six counting Concentricycloidea) are alive today:

  • Asteroidea (asteroids, starfish, or sea stars): about 1,500 species that capture prey for their own food.
  • Concentricycloidea (sea daisies), notable for their unique water vascular system; two species; recently merged into Asteroidea.
  • Crinoidea (crinoids, feather stars or sea lilies): about 600 species that are suspension feeders.
  • Echinoidea (echinoids, sea urchins and sand dollars): notable for their movable spines; about 1,000 species.
  • Holothuroidea (sea cucumbers): elongated animals resembling slugs; about 1,000 species.
  • Ophiuroidea (brittle stars and basket stars), the physically largest of echinoderms; about 1,500 species.

Extinct forms known from fossils include blastoids, edrioasteroids, and several early Cambrian animals such as Helicoplacus, carpoids, Homalozoa, and possibly machaerids.

Echinodermata is the largest animal phylum to lack any freshwater or terrestrial representatives.

Mollusca — squid, snails

The mollusks (American spelling) or molluscs (British spelling) are the large and diverse phylum Mollusca, which includes a variety of familiar animals well-known for their decorative shells or as seafood. These range from tiny snails, clams, and abalone to squid, cuttlefish and the octopus (which is considered the most intelligent invertebrate). There are some 112,000 species within this phylum.[2]

The giant squid, which until recently had not been observed alive in its adult form, is the largest invertebrate; although it is possible that the colossal squid is even larger. The scientific study of mollusks is called malacology.


Arthropoda — insects, ticks, spiders, grasshoppers, lobsters, crabs

Arthropods (Phylum Arthropoda, from the Greek ἀρθρον, meaning joint and πούς/ποδός, meaning foot) are the largest phylum of animals and include the insects, arachnids, crustaceans, and others. More than 80% of described living animal species are arthropods [3], with over a million modern species described and a fossil record reaching back to the early Cambrian. Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (~¼ mm) up to forms several metres long.

Arthropods are characterised by the possession of a segmented body with appendages on each segment. They have a dorsal heart and a ventral nervous system. All arthropods are covered by a hard exoskeleton made of chitin, a polysaccharide, which provides physical protection and resistance to desiccation. Periodically, an arthropod sheds this covering when it moults.


  • Bryozoa — moss animals, sea mats (occasionally resemble corals)

Bryozoans are tiny colonial animals that generally build stony skeletons of calcium carbonate, superficially similar to coral. They are also known as moss animals or sea mats. They generally prefer warm, tropical waters but are known to occur worldwide. There are about 5,000 living species, with several times that number of fossil forms known.


  • Placozoa (trichoplax)
  • Orthonectida (orthonectids)
  • Rhombozoa (dicyemids)

Subregnum Parazoa

Subregnum Eumetazoa



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  1. ↑ Dalby, Andrew [2003]. "Sea anemone", Food in the Ancient World: from A to Z (in English). London, New York: Routledge, 296. 
  2. ↑ Feldkamp, Susan (2002). Modern Biology. United States: Holt, Rinehart, and Winston. pp. 725
  3. ↑ Anna Thanukos. The Arthropod Story. University of California, Berkeley.
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