Size range and diversity of structure
Protozoans range in diameter from a few thousandths of a millimetre to several millimetres. Because the group contains many unrelated or loosely related organisms, enormous diversity in structure and form exists.
Flagellated protozoans
The flagellated protozoans range from a simple oval cell with one or more flagella to the structural sophistication of the collared flagellates (choanoflagellates, supergroup Opisthokonta). The collared flagellates lack photosynthetic pigments and are therefore colourless. They have a single flagellum surrounded by a delicate circular collar of fine pseudopodia (microvilli) on which they trap food particles. In some marine species the whole cell is enclosed in an elaborate, open latticelike basket formed from strands of silica. Although some dinoflagellates (supergroup Chromalveolata) still contain plant pigments and rely to a greater or lesser degree on photosynthesis, many members have lost the ability to photosynthesize. All dinoflagellates are surrounded by a cell wall armour with a complicated pattern and possess two flagella, one of which beats in a transverse plane around the equator of the cell while the other beats in a longitudinal plane.
Many other flagellated protozoans can develop stalks that connect them to a substrate, either as single cells (e.g., the genus Paraphysomonas) or as colonies (e.g., the genus Codosiga). Other flagellated taxa may exist as swimming colonies; for example, in Sphaeroeca and Spongomonas many individual flagellated organisms are embedded in an unstalked gelatinous sphere.