Classification of the Animal Kingdom
The animal kingdom (formal classification "Kingdom Animalia") is the most diverse of the four eukaryotic kingdoms, and also the most familiar. Containing over 2 million known species and millions more suspected, the animal kingdom contains organisms that thrive in virtually every environment, including the bottom of the ocean, the middle of the desert, and deep within rain forests. They're also very well-known to us - we ourselves are members of the animal kingdom, as are many of the organisms that we eat, keep as pets, and exterminate as pests. However, many less well-known organisms are part of the animal kingdom as well, for example worms, leeches, and sponges.
The animal kingdom contains such an incredible variety of species that they couldn't possibly be covered together on one page. However, there are some basic characteristics that all animals share. Formally, an animal is defined as any multicellular organism whose cells are enclosed only by a plasma membrane and not by a cell wall. This definition separates animals from protists, which are generally single-celled, and from fungi, which have cell walls. In addition, all animals are heterotrophic, consuming other organisms to provide themselves with energy. This separates animals from plants, which are autotrophic. In terms of reproduction, all animals start life as haploid gametes, but mature into diploid organisms. As well, all animals have the capacity to reproduce sexually, although many also reproduce asexually.
The animal kingdom is divided into 33 phyla. However, most of the phyla are composed of extremely esoteric species. For the sake of brevity and interest, this page will only cover the nine largest animal phyla: porifera, which contains all sponges; Cnidaria, which contains all jellyfish and sea anemones; Plathelminthes, which contains all flatworms; Nematoda, which contains all roundworms; Mollusca, which contains all mollusks; Annelida, which is made up of segmented worms; Arthropoda, which contains all arthropods; Echinodermata, which contains starfish and sea urchins; and chordata, which contains all animals containing a spinal cord.
To access more specific information on each kingdom, click on the picture of the appropriate phylum below: