If this apparatus of fluid contained in vesicles and canals constitutes a necessary and constant integer of these organisms, it undoubtedly implies a great complexity of structure. Such organisms are, therefore, not unicellular, but policellular. But compare a protozoon with an acknowledged cell—what a cell is, it is not easy to explain.
--Thomas Williams, 1856, The British and Foreign Medico-chirurgical Review, vol. 18, p. 464