Gastrointestinal tract simulation system and compartment therefor. The compartment comprising a vessel having an open top surrounded by a peripheral edge portion and an air-tight lid system configured to be placed onto the peripheral edge portion. The lid system comprises a body with a plurality of passageways extending through the body and providing access to the interior of the vessel, the plurality of passageways comprising passageways for fluid transfer tubes and passageways for mounting at least one sensor component. The lid system is provided with releasable sealing elements for sealing the plurality of passageways and at least one pressing element which is common to at least a number of the sealing elements and configured for applying pressure to each of the respective sealing elements to effect the sealing of the respective passageways.