a pouch or sac without opening, usually membranous and containing morbid matter, which is accidentally developed in one of the natural cavities or in the substance of an organ.-in old authors, the urinary bladder, or the gall bladder.-one of the bladders or air vessels of certain algae, as of the great kelp of the pacific, and common rockweeds (fuci) of our shores.-a small capsule or sac of the kind in which many immature entozoans exist in the tissues of living animals; also, a similar form in rotifera, etc.-a form assumed by protozoa in which they become saclike and quiescent. it generally precedes the production of germs. see encystment.