Kingdom Metazoa

The Kingdom Metazoa includes all multicellular animals. The unicellular animals have been placed in the phylum protozoa which is now a part of the kingdom protista. Thus, all the remaining animals from sponges to mammals are now placed in the Kingdom Metazoa.

Of the nearly million identified species of animals, more than about 95% are described as invertebrates, since they do not have a vertebral column (or backbone). These animals exist in a wide variety of sizes and shapes ranging from those which are only a fraction of a millimeter in length to those giant forms which may be several meters in length. They occur in a diverse range of habitats and exhibit an extraordinary range of life styles. Apart from the invertebrates, the only other major group of animals is that of chordates animals with a solid supporting structure called notochord, at some stage of their life cycle. The chordates include mainly the vertebrates which have a backbone. Vertebrates represent less than 5% of the known animal species.

A detailed discussion on the general characteristics of animals, their classification and examples, is given in the chapter - The Animal kingdom.