The octopus is used for the calibration of the pressure gauges of the ISR. It was part of a system which made it possible to calibrate at very low pressures (from 10$^{-6}$ to 10$^-{12}$ torr). The large aperture was connected to a cryopump and on the opposite side gases (such as hydrogen or nitrogen) were introduced in a controlled way. The pressure gauges were connected to the eight side arms. With two of these devices on the same pump, sixteen gauges could be set up for calibration at the same time.