A chamber having the shape of a tube inside the valve, oriented along the apical axis. It occurs in genera such as Neidium and Muelleria, which have valves consisting of two layers of silica with the canal lying between them. The canals appear as longitudinal lines and may be central or marginal.
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.