Before revision: A "critical siphon" is a subset of the species in a chemical reaction network whose absence is forward invariant and stoichiometrically compatible with a positive point. We define "catalytic networks " as networks  with an essentially catalytic reaction pathway: one which is "on " in the presence of certain catalysts and "off " in their absence.
After revision: We define catalytic networks with an essentially catalytic reaction pathway: one which is "on " in the presence of certain catalysts and "off " in their absence.