The rigid or semi-rigid envelope lying outside the cell membrane of plant, fungal, most prokaryotic cells and some protozoan parasites, maintaining their shape and protecting them from osmotic lysis. In plants it is made of cellulose and, often, lignin; in fungi it is composed largely of polysaccharides; in bacteria it is composed of peptidoglycan; in protozoan parasites such as Giardia species, it's made of carbohydrates and proteins. [ http://en.wikipedia.org/wiki/Microbial_cyst https://www.worldcat.org/search?q=bn%3A0198547684 http://www.ncbi.nlm.nih.gov/pubmed/15134259 ]