There are three levels of source material. It is safest to use only primary sources. The further you get from these sources, the greater the risk of reporting inaccurate information. Primary sources are the original articles based on the original researcher's work. Secondary sources cite information from primary sources in textbooks or review articles.
Tertiary sources collect information from secondary sources for encyclopedias and dictionaries.
Bettica, A. 1988. The presence of Chlamydial organisms in the hepatopancreas of the terrestrial isopod, Porcellio scaber , as revealed by on-grid localizations., in press. Bettica, A., Shay, M. T., Vernon, G. M. and Witkus, E. R. 1984. An ultrastructural study of cell differentiation and associated acid phosphatase activity in the hepatopancreas of Porcellio scaber. Cicero, R. 1964.