Actin puts the A in Animal.  It’s a good part of the steak, for one thing: actin is most familiar in its first-discovered role as the lattice of filaments that make up muscle.  Among the early biological discoveries of the electron microscope was that similar filaments are almost everywhere in animal cells.  Other eukaryotes also depend heavily on the assembly of actin into filaments that are then combined into various structures, but it is hard to find another group that uses actin so pervasively as the animals.  As depicted (faintly) in the videos on this page, the animal cell cleavage furrow is based on assembly and (supposedly) constriction of an actin-based contractile ring.  But even more, animal cells rely, to a greater extent than many other eukaryotes, on actin to give cells shape.  Where green plants, for instance, might be bounded by an extracellular wall of cellulose, or fungal cells by chitin, animal cells often use a meshwork of actin filaments to give shape and possibly tensility to the cell cortex, a structured layer of cytoplasm just beneath the plasma membrane.

In the videos on this page actin filaments are labeled with GFP fused to the actin-binding calponin homology domain from human Utrophin.  This domain specifically binds filaments, not the unassembled monomers.  The dominant actin-based features in these early urchin embryos are the microvilli, small projections which make a microscopic forest above the cell surface.  These extend and retract partially as cells divide, and the actin of the cleavage furrow appears beneath them.