The lack of a rigid cell wall allowed animals to develop a greater diversity of cell types, tissues, and organs. The ability to move about by the use of specialized muscle tissues is the hallmark of the animal world. The animal kingdom is unique amongst eukaryotic organisms because animal tissues are bound together by a triple helix of protein, called collagen. Plant and fungal cells are bound together in tissues or aggregations by other molecules, such as pectin. The fact that no other organisms utilize collagen in this manner is one of the indications that all animals arose from a common unicellular ancestor.
Animals are a large and incredibly diverse group of organisms.
Being mobile has given animals the flexibility to adopt many different modes of feeding, defense, and reproduction. The first mass extinction ended that period, but during the Cambrian Period which followed, an explosion of new forms began the evolutionary radiation that produced most of the major groups, or phyla, known today. They appear to help in organizing cell division, but aren't essential to the process. In multicellular organisms, cilia function to move fluid or materials past an immobile cell as well as moving a cell or group of cells. Endoplasmic Reticulum - The endoplasmic reticulum is a network of sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell.