The thin, semirigid, transparent outer covering that protects and maintains the shape of certain protists, e.g. Euglena. In ciliated organisms, e.g. Paramecium, it consists of regular arrays of membranous sacs, called alveoli. The centre of each sac forms a depression, arising from which are one or two cilia. Beneath the pellicle are tracts of contractile filaments called kinetodesmata. These connect to the bases of the cilia, and to each other, to form a supporting cytoskeletal mesh that can alter the shape of the cell. Defensive organelles called trichocysts project between the alveoli. These can discharge a long thread with a harpoon-like tip. See also alveolates.