What are invertebrate animals?
We explain which are the invertebrate animals and how these animals are classified. In addition, examples of invertebrates.
Invertebrates represent 95% of known living species.
Invertebrate animals are those species of the animal kingdom that do not have a notochord or dorsal cord, nor a vertebral column, nor an articulated internal skeleton. In this group are 95% of known living species, between 1.7 and 1.8 million species (2005 figures).
Invertebrates also tend to be smaller than large terrestrial or aquatic vertebrates, and although they lack an articulated skeleton, they often have exoskeletons (such as insects) or shells and shells of resistant substances (such as molluscs).
This is by no means a homogeneous group: from the beginning of the term “invertebrate” (from the French animaux sans vertèbres, animals without vertebrae), its creator, the naturalist Jean-Baptiste Lamarck, subdivided them into ten different classes: molluscs, barnacles, annelids, crabs, spiders, insects, worms, echinoderms, polyps and infusoriums. This classification is no longer used, of course.
The group of invertebrates has been difficult to classify and study, given their small size and the diversity of environments they inhabit. In the current zoological classification, it is estimated that the group is composed of animals with the following edges:
- Arthropods, endowed with exoskeletons and articulated limbs, such as insects, arachnids, crustaceans and myriads.
- Molluscs, soft-bodied and not segmented, often covered with shells or shells to defend it.
- Poriferous, that is to say, marine sponges, with bodies endowed with radial symmetry.
- Cnidarians, simple aquatic animals such as corals and jellyfish.
- Echinoderms, mostly marine, such as sea urchins and starfish.
- flatworms (in some cases parasites).
- Nematodes, or cylindrical worms.
Annelids, or earthworms and leeches.