All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix types a relatively flexible framework upon which cells can transfer about and be reorganised, making the formation of complicated constructions potential. This may be calcified, forming structures corresponding to shells, bones, and spicules. In distinction, the cells of other multicellular organisms are held in place by cell partitions, and so develop by progressive progress.

Unlike vegetation and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. All animals are motile during a minimal of part of their life cycle, but some animals, such as sponges, corals, mussels, and barnacles, later turn out to be sessile. The blastula is a stage in embryonic growth that’s distinctive to animals, permitting cells to be differentiated into specialised tissues and organs. Historically, …