Invertebrate - Taxonomic Significance

Taxonomic Significance

The term Invertebrate generates a great deal of confusion among non-biologists; it does not refer to any particular taxon in the same way that for instance Arthropoda, Vertebrata or Manidae do. Each of those examples describes a (presumably monophyletically) valid taxon, say a phylum, subphylum or family. In referring to taxonomy of the Animalia, "invertebrata" is a term of convenience, not a taxon; it has very little circumscriptional significance except arguably within the Chordata. The Vertebrata as a subphylum comprises such a small proportion of the Metazoa that to speak of the kingdom Animalia in terms of "Vertebrata" and "Invertebrata" would be about as practical as classifying animals into mayflies and non-mayflies, or transport into rowing boats and non-rowing boats. It would be logically correct to do so, and rowing boats as such do form a practical group, but speaking of "non-rowing boats" would lump together land, sea, air and space transport in ways that rarely would be useful. In formal taxonomy of the Animalia there are higher level attributes that logically should precede the presence or absence of the vertebral column in constructing a cladogram, for example, the presence of a notochord. That would at least circumscribe the Chordata. However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry or perhaps bauplan. The resultant cladistic structure would not resemble anything like a binary split into vertebrates and invertebrates.

At the same time, there certainly is no reason to avoid the use of the terms Invertebrata or invertebrates when they are convenient, but it is important not to confuse the terms with the names or relationships of actual taxa.

Of the million or more animal species in the world, more than 98% are invertebrates. Invertebrates don't have a skeleton of bone, either internal or external. They include hugely varied body plans. Many have fluid-filled, hydrostatic skeletons, like jellyfish or worms. Others have hard exoskeletons, outer shells like those of insects and crustaceans. The most familiar invertebrates include the Protozoa, Annelida, Echinodermata, Mollusca and Arthropoda. Arthropoda include insects, crustaceans and arachnids.