Structure
All animals have eukaryotic cells, enclosed by a characteristic extra cellular matrix collected of collagen and stretchy. This may be calcified to form structures like shells, bones, and spackles. Throughout development it forms a comparatively flexible framework upon which cells can move about and be rationalized, making complex structures possible. In contrast, other organisms like plants and fungi have cells detained in place by cell walls, and so expand by progressive growth. Also, unique to animal cells are the next intercellular junctions: tight junctions, gap junctions.