Why can animal cells be of various shapes, while cells of other kingdoms have a strictly defined shape?

Animal cells are able to change shape for the reason that they do not contain membranes in their composition. The unchanging form of representatives of other kingdoms is due to the presence of rigid cell membranes: in plants they are represented by cellulose, in fungi – by chitin (sometimes they also contain cellulose), and in bacteria – by polysaccharides, the main component of which is murein.