The mycoplasma species and the L-forms are two main examples of wall-less bacteria. Mycoplasma possesses a surface membrane structure in place of a cell wall. The smallest and simplest of free-living prokaryotes, mycoplasma are able to assume different forms because of the absence of a cell wall. L-form bacteria are a form of bacteria that have lost their ability to produce peptidoglycan structures, which are an essential component of the cell wall. They can arise from gram-positive, gram-negative bacterial cells, and are pleomorphic.  A membrane or exoskeleton separates them from the environment inside the cell and protects them from being digested by the cell.