The long axis of a diatom is considered the apical axis.
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.
Image Credit: Marina Potapova
Navicula is symmetrical to the apical axis. The black line runs along the apical axis.