The animal kingdom is extremely diverse, with some animals being microscopic while others are several meters long, living in various environments from inside other bodies to the open sea. Animals are multicellular eukaryotes that lack cell walls and carry out essential functions like feeding, respiration, circulation, excretion, response, movement, and reproduction to maintain homeostasis. The kingdom is divided into invertebrates, which lack backbones, and vertebrates, which have backbones.