All animals are heterotrophic. And all animals respond to external stimuli. All animals are motile, moving about at some point in their life cycle. Only animals can fly. J.B.S. Haldane once called animals wanderers in search of spare parts.
All animals reproduce sexually by forming haploid gametes of unequal size, the egg and the sperm.
The gametes fuse into a zygote , which develops into a hollow ball of cells called a blastula . In most animals, this hollow ball folds inward to form a gastrula , a hollow sac with an opening at one end, the blastopore , and an interior space or blastocoel . Most authorities now agree that all animals share a common ancestor, most probably some type of colonial protozoan called a choanoflagellate, perhaps a protozoan version of Volvox . We can divide the entire animal kingdom into two subkingdoms. The Subkingdom Parazoa contains the sponges, and one or two obscure groups of rudimentary animals.