Piezoelectric glass?

[Aggressive_Doughnut]

I've been starting to get into playing with piezo devices, but I keep seeing this one diagram for a device that confuses me.  Here you'll see a liquid density measuring device  diagram and it includes a piezo actuator.  As the actuator is outside of the enclosing tube, I can only think that the device somehow interacts with the internal glass tube.  Is this right? or is the drawing incorrect somehow?