Protozoans - Body Form
The protozoan body is usually bound only by the cell membrane called plasmalemma. In some protozoans such as in ciliates the rigidity and the flexibility of the body is maintained by a cytoskeleton located immediately below the cell membrane which together with it forms the pellicle. The cytoskeleton is composed of filamentous proteins, microtubules or vesicles or a combinations of all three. In many protozoans like dinoflagellates, spore forming protozoans and. ciliates, vesicles occur below the cell membrane as flattened more or less continuous layer referred to as alveoli. They help to form a rigid skeleton.
Figure: Endoskeletons of Protozoans
A) Filamentous proteins form dense mesh called epiplasm Eg. Euglena and some ciliates B) Micro tubular corset in flagellates, in spore forming protozoa and some ciliates C) Axial skeleton of micro tubes D) Axopod microtubules radiate like stars from centroplast E) 'Empty' alveoli of ciliates F) Cellulose plates in alveoli of dinoflagellates form rigid skeleton.
Exoskeletons secreted onto the outer surface of the cell are also seen in a large number of protozoans. These protozoans secrete layers of certain organic materials like gelatin, cellulose or tectin (a mucus like substance) around their plasmalemma. Some secrete an organic matrix containing minerals like silica or calcium carbonate. Often foreign bodies such as sand particles also get mixed up with the organic matrix to strengthen it. These chemicals organise to form theca, shell, lorica or cyst in different protozoans to provide protection.