Invertebrate

Invertebrate

Invertebrates are animal species that do not develop a vertebral column. This in effect includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include insects, worms, clams, crabs, octopus, snails, and starfish. Taxonomically speaking "invertebrate" is no more than a term of convenience. The vast majority of animal species are invertebrates, since only about 3% of animal species include a vertebral column in their anatomy. In other words all animals except those in the chordate subphylum Vertebrata (fish, amphibians, reptiles, birds, and mammals) are regarded as invertebrates. What is more, many an individual invertebrate taxon has a greater number and variety of species than the entire subphylum Vertebrata, In fact some of the so-called invertebrata, such as the Chaetognatha and Hemichordata, are more closely related the Chordata than to other invertebrate phyla. The division of the entire Kingdom Animalia into vertebrates (about 40000 species in part of one phylum) and invertebrates certainly is convenient in some practical contexts, but to put it into taxonomic perspective, it is roughly on the same scale as dividing the animal kingdom into Gastropoda (perhaps 60000 species in part of one phylum) and non-Gastropoda; worthwhile only in certain constrained contexts.

Invertebrates accordingly form a massively paraphyletic group. It is generally accepted that the phyla comprising modern Eukaryota share a common multicellular ancestor, but with the sole exception of one subphylum of the phylum Chordata, all those phyla are classified as invertebrates along with two of the three subphyla in the Chordata: Tunicata 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. Within palaeozoology and palaeobiology, invertebrates are often studied within the fossil discipline called invertebrate palaeontology.