What is the part called that sits on top of the thermostat and is hooked to the hose on a dodge stratus?

Noticed today there is a few pin holes in the spout? that sits on top of the thermostat and the hose from the rad goes into it. I had the head gasket done a few weeks agao and wasn't getting much heat and has a new thermostat..What could of caused the pin holes and just luke warm heat. Wrong thermostat? The heat gauge has been normal but just not much heat and now i noticed the pin holes..Any help..Thanks