Comprising organisms which develop from two different haploid gametes, a small sperm and a larger egg cell, and go through a blastula-stage in their ontogenetic development. Animals are heterotroph organisms which obtain energy by consuming other organisms, either plants or other animals. Because of their heterotrophy animals are usually free-living and mobile, while sessile animals are capable of moving feeding structures or can retract in a protective shell or tube. Some animals, such as internal parasites, have completely lost the ability for movement. The wall of animal cells is devoid of any rigid material. Animals have developed sensory organs and a nervous system for transmission and processing of external stimuli. Usually animals have a digestive system, which may have only one opening or may have also another opening that functions as anus. Most animals possess an excretory system for osmotic pressure balance and to eliminate nitrogenous waste products.
The kingdom Animalia is one of the five kingdoms in which life on earth is classified, the others being Plantae, Fungi, Protoctista, and Prokaryotae. Animalia is divided into the following major groups or phyla: Placozoa (one species: Trichoplax adhaerens), Porifera (Sponges), Cnidaria (Coelenterates), Ctenophora (Comb jellies), Mesozoa (Mesozoans), Platyhelminthes (Flatworms), Nemertina (Ribbon worms), Gnathostomulida (Gnathostomulids), Gastrotricha (Gastritrichs), Rotifera (Rotifers), Kinorhyncha (Kinorhynchs), Loricifera (Loriciferans), Acanthocephala (Spiny-headed worms), Entoprocta (Entoprocts), Nematoda (Nematodes), Nematomorpha (Horsehair worms), Ectoprocta (Ectoprocts), Phoronida (Phoronids), Brachiopoda (Lamp shells), Mollusca (Mollusks), Priapulida (Priapulids), Sipuncula (Peanut worms), Echiura (Spoon worms) Annelida (Annelids), Tardigrada (Water bears), Pentastoma (Tongue worms), Onychophora (Velvet worms), Arthropoda (Arthropods), Pogonophora (Beard worms), Echinodermata (Echinoderms), Chaetognatha (Arrow worms), Hemichordata (Hemichordates), Chordata (Chordates).