Pacer Gas Tank area - What is it?

Changing the sending unit in my Pacer gas tank has led me to having the tank cleaned and red coated and changing all the small rubber hoses (gas and vent).  Can someone tell me what the little aluminum can is that seems to be related to the air venting? There are two vent lines, but the one with the can is on the line coming from the actual gas tank sending unit.  

Thanks for the answer.  I googled "rollover vent" to see a picture and that's it.  I'm going to try and change its hoses as they are somewhat dry. Would there be any recommended service to the unit? 

Thanks Joe