A small perforation through the valve. Plural form is porelli. Groups of closely packed porelli occur in ocelli, pseudocelli and apical pore fields.
Ross, R. and Sims, P. (1972). The fine structure of the frustule in centric diatoms: a suggested terminology. British Phycological Journal 7: 139-163.
Round, F.E., Crawford, R.M. and Mann, D.G. (1990). The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge, 747 pp.