Theca - body form Protozoans
It is a close-fitting hard structure around the plasma membrane, mainly composed of cellulose. It can be compared with the ceil wall of a plant cell. Theca can be one to many layers thick but still remains flexible; sometimes it may become rigid after getting impregnated with inorganic salts. The surface of theca can be smooth and plain or rough and ornamented e.g. Glenodium.
Figure:Ornamental theca in Glenodium