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WHAT IS KINGDOM ANIMALIA?
Kingdom Animalia is one of four kingdoms in the Domain Eukarya. It is distinct from the other three kingdoms, Plantae, Fungi, and Protista, in several ways. Animalia are multicellular, while most Protista (excepting the multicellular algae, which are plant-like) are unicellular. Heterotrophism separates the animals and fungi from plants, and the lack of cell walls in animal cells makes them distinct from fungi. Animals also possess several other unique features. These include interior digestion of food, possession of a digestive tract where hydrolytic enzymes are secreted and digestion takes place, and special cell junctions in their tissues.

The life cycle of organisms in Kingdom Animalia also separates them from organisms in the other three kingdoms. Animals spend their entire life cycle as diploid cells, with the exception of haploid gametes. The first stage of their life is as haploid reproductive cells (sperm and eggs) in the mature adult organisms. The gametes fuse to form a zygote. They zygote then undergoes mitotic divisions, which lead to a stage of development called the blastula. The blastocyst (blastula structure) consists of a single cell layer around a fluid-filled cavity. The formation of a gastrula, by infolding of the blastocyst in a blastopore, is also common to most animals. A gastrula consists of an inner and outer cell layer. The outer layer usually becomes the epidermal and nerve cells of the adult organisms--the ectoderm. The inner layer becomes the digestive tract, or endoderm. A third layer-the mesoderm-usually infolds, and develops into the other internal organs. From this stage, some animals develop into larva, which are immature specimens appearing very different from the adult. Larva then undergo a metamorphosis in which they become a mature adult, capable of reproducing.

Kingdom Animalia is thought to have arose in the sea, from colonial protists.


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