An animal is a multicellular heterotrophic eukaryote - heterotropic in the detritic (organic material in a state of decomposition) sense. An animal has carbohydrate reserves in the form of glycogen. Animals lack cell walls and therefore have unique intercellular junctions. Two unique tissues for movement and impulse are the nerve and muscular tissues. Finally, animals (most of them anyway) reproduce sexually: sperm and ova unite to form a zygote; then through cleavage it becomes a morula, a cluster of cells, then a blastula, a hollow sphere of cells. Then in a process called either gastrulation or invagination some of the cells of the blastula get ingested and form a "blastula in a blastula" so to speak. This new arrangement of embryonic cells is called a gastrula. As an animal matures its development can be a continuous progression or a series of steps divided into distinct (larval) stages; either way this is called metamorphosis.

Next: "Invertebrate Phylogeny."