It is distinct from the other three kingdoms, Plantae, Fungi, and Protista, in several ways. 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 gametes fuse to form a zygote. They zygote then undergoes mitotic divisions, which lead to a stage of development called the blastula. 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 inner layer becomes the digestive tract, or endoderm.