Extendible and retractable endoscopic cap

A device for treating tissue, comprising:

• a handle including a first actuator;
• a first control cable extending distally from a proximal end coupled to the first actuator;
• and
• a cap including a mounting structure configured to be removably coupled in an operative configuration on a distal end of an insertion device, the insertion device being configured to be inserted to a target site within a living body,
• wherein the cap further includes an extending part movably coupled to the mounting structure for movement between a retracted position in which a distal end of the extending part is received over the distal end of the insertion device and an extended position in which a distal end of the extending part extends distally beyond the distal end of the insertion device, the extending part including a wall defining an internal channel so that, when the extending part is in the extended position, the wall surrounds a space contiguous to the distal end of the insertion device, the extending part being coupled to a distal end of the first control cable so that, actuation of the first actuator moves the extending part between the extended and retracted positions.