ANIMAL KINDOM - 3 Structure- Reproduction and development- Ecology - Diversity - Size - Numbers and habitats of major phyla - Evolutionary origin - External phylogeny All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins . [ 23 ] During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as shells , bones , and spicules . [ 24 ] In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi ) are held in place by cell ...