As the name suggests, ‘invertebrates’ are organisms that are not vertebrates, meaning they lack a vertebral column or backbone, unlike their counterparts. It is more of an informal group, including the phyla arthropods, mollusks, annelids, echinoderms, flatworms, cnidarians, and sponges, except those belonging to the subphylum Vertebrata (paraphyletic).
Almost 97% of all animal species are invertebrates (May, Robert M.1988), and the IUCN estimates over 1,300,000 described species.



While the smallest invertebrates in the world, the Myxozoans, measure around 10 to 25 µm during adulthood, the largest, the giant squid, grows up to 60 feet (18.2 m) in length.

Body Plan

Although the bodies of most invertebrates are radially, bilaterally, or spherically symmetrical, a few, like gastropods, corals, and sponges, show a complete lack of symmetry. 

Each phylum of invertebrates exhibits different patterns of body segmentation, arrangements of body cavities (coeloms), and the presence of internal organs.


  • The members of the phylum Porifera (sponges) have differentiated cells that are not organized into distinct tissues.
  • The phyla Ctenophora (comb jellies) and Cnidaria, including sea anemones, corals, and jellyfish, are characterized by radially symmetrical bodies and digestive chambers with a single opening, which serves as both the mouth and the anus.
  • Echinodermata, including starfish, sea urchins, sea cucumbers, and their relatives, are characterized by pentamerous (five-pointed) radial symmetry.
  • Platyhelminthes, or flatworms, are acoelomates with body cavities (coelom) completely reduced, like those of the phylum Gastrotricha (hairybellies or hairybacks.)
  • The largest animal phylum, Arthropoda, includes insects, crabs, spiders, and their relatives. These have jointed paired appendages and segmented bodies covered by a hard exoskeletal cuticle that they shed periodically.
  • Mollusca is the second-largest animal phylum, containing soft-bodied, asymmetrical invertebrates like snails, clams, and squids, whereas Annelida includes segmented worms, such as earthworms and leeches.
  • Smaller phyla, such as Kinorhyncha, Priapulida, and Loricifera, are pseudocoelomates characterized by reduced coelom.

Organ System

  • Most invertebrates have an open circulatory system, in which body fluid flows freely through cavities, and there are no specific blood vessels for circulation. However, annelids possess closed circulatory systems, in which networks of minute capillaries unite the arteries with the veins.
  • While most terrestrial invertebrates breathe through the trachea (open respiratory system), aquatic dwellers use gills for respiration. The tracheae are invaginations of the cuticular spiracles (openings) that branch into minor tubules called tracheoles, performing gas exchange through diffusion. Some terrestrial invertebrates, like the pulmonate land snail, use lungs for respiration.
  • They typically have a single digestive chamber that opens to the exterior.
  • Invertebrates have a simple and mostly decentralized nervous system comprising nerve nets, ganglia, and nerve cords arranged differently across groups.
  • Jellyfish, sea stars, flatworms, and ribbon worms have the simplest eyes, called pigment spot ocelli, which lack a cornea, lens, or retina. In contrast, arthropods, like insects and crustaceans, have well-developed compound eyes comprising thousands of tiny photoreception units called ommatidia.
  • Unlike vertebrates, they possess specialized auditory organs that can sense sounds. For example, the female cricket fly Ormia ochracea has membranous tympanal organs on each side of her abdomen that detect sound vibrations in the surroundings. Similarly, spiders and cockroaches bear sensory hairs on their limbs.


In 1973, French naturalist Jean-Baptiste Lamarck coined the term ‘invertebrates’ as an umbrella term encompassing all non-vertebrate animals. However, it does not refer to any particular taxon as Vertebrata.

Carl Linnaeus initially divided invertebrates into two broad groups: the Insects and the obsolete Vermes (Worms). Later, Lamarck further subdivided the Linnean Insects into Arachnida and Crustacea and the Linnean Vermes into Mollusca, Annelida, Cirripedia, Radiata, Coelenterata, and Infusoria. 

There are currently over 30 phyla of invertebrates, with insects being the largest group and over 1,000,000 characterized species. The following list shows some of the different groups of invertebrates and their members.

  • 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 (Extinct) (Hemichordata)
  • Lancelets (Amphioxiformes)
  • Salps, pyrosomes, doliolids, larvaceans, and sea squirts (Tunicata)
  • Cycliophora



Scientists believe animals originated around 1 billion years ago, but multicellularity evolved during the Tonian Period (1000 to 720 million years ago). Some 665-million-year-old fossils unearthed from the Trezona Bore, South Australia, have been proven as early sponges.

Animals began diversifying around 453 million years ago, and with that began the diversification of invertebrates accompanied by their increasing number.

Distribution and Habitat

They are cosmopolitan in distribution and occupy almost every habitat, including land, seas, and freshwater bodies like lakes and rivers.


While the shortest-living American sand-burrowing mayfly barely lives for 24 hours, the longest-living invertebrate, the ocean quahog (Arctica islandica), has survived up to 507 years.


Termites, bees, ants, and other vertebrates exhibit eusociality, living in groups and displaying social behavior, like division of labor among the members.


Invertebrates mostly reproduce sexually by producing haploid gametes that fuse to form a diploid zygote (fertilization). Except for cephalopods and arthropods, all invertebrates are hermaphroditic and undergo external fertilization.

However, hydra, planarians, and a few annelids reproduce asexually through modes like budding or fission.

Additionally, sea anemones and polyps of corals reproduce both sexually and asexually.

Interesting Facts

  • The first invertebrate in which a neuron cell was identified is Hirudo medicinalis (Medicinal Leech).
  • The study of invertebrates was not popular until Linnaeus and Lamarck delved deeper into their biology. During the 20th century, the visibility of invertebrate zoology as a subject improved, and it is one of the major streams of natural sciences today.


  1. Invertebrate: 
  2. Invertebrate: 
  3. Invertebrates: 
  4. Invertebrates:
  5. What is an Invertebrate?:
  6. Nervous System of Invertebrates: