Number and Phyla
At present there are probably about 2 million species of animals. Of these only half have been formerly described. Of these approximately 8 per cent are the protistan, by about 3-5 per cent are vertebrates and about 90 per cent are constituted by invertebrates.
Invertebrates are grouped into about 30 phyla. The present day invertebrates are calculated to be just one-seventh of the known species, the rest of them are extinct.
Major and Minor Phyla
Of the 30 phyla of invertebrates, some of them have been more successful than others with respect to the number of species. It is customary to regard them as major phyla. The rest are considered to be minor phyla. The classification as major and minor phyla is based on the number of 'individuals' in a phylum, of the 'successful dominant' species in the group or on the evolutionary significance of the phylum. Generally nine phyla are considered to be the major phyla. These are Protozoa, Porifera, Coelenterata, Platyhelminthes, Nemathelminthes, Mollusca, Annelida, Arthropoda, and Echinodermata. The rest of the invertebrates constitute the minor phyla.
The use of the terms lower invertebrates and higher invertebrates is also very common in invertebrate classification. These terms refer to the overall structural complexity and the extent of functional integration in each phylum. The lower invertebrates are simpler in body organisation and are generally, small in size. Protozoans, Poriferans, Coelenterates, Platyhelminthes, and Nematodes belong to the lower invertebrates. Higher invertebrates include the phyla Annelida, Arthropoda, Mollusca and Echinodermata.
All the phyla of invertebrates probably originated in the sea. However, only some of them have successfully invaded the land or its fresh water habitats. Some phyla such as Echinodermata, Brachiopoda, etc., have never successfully colonised fresh waters; other phyla such as Porifera and Coelenterata are represented in fresh waters as well as sea waters; more species of them are found in sea water. The major groups both in terms of number of individuals as well as number of species, found in fresh waters are insects and rotifers. Earthworm is a terrestrial animal belonging to Annelida; but most of the annelids are marine. Arthropods are the most successful terrestrial invertebrates, excepting crustacea which are largely marine, the arthropods such as insects, spiders, centipedes and millipedes are mostly terrestrial.