on its inner surface at a point intermediate the radial ports 34 and 36 with an upwardly facing ball seating surface 38 when a ball 60 fig2 a is dropped through the tubing string after the perforating gun has been discharged such ball will seat and seal on the ball seating surface 38 and thus effectively block the lower radial port 34 from communication with the fluid pressure existing in the tubing string hence an increase in fluid pressure in the tubing string produced from the surface will increase the fluid pressure above the annular piston 50 relative to the fluid pressure existing below the annular piston 50 when this fluid pressure is increased to a level sufficient to effect the shearing of shear pin 52 the piston 50 moves downwardly thus removing its upper portions 50a from engagement with the collet surface 12d and permitting the latching heads 12b of the collet 12 to be deflected inwardly it should be noted that if the weight of the perforating gun is not sufficient to effect such deflection of the collet latching heads 12b the downward movement of the piston 50 will result in such piston striking the upwardly facing bottom surface 35d of the fluid pressure chamber 35 and will produce a downward force on the entire coupling apparatus sufficient to effect the disengagement of the collet latching heads 12b from the annular latching surface 40c accordingly as soon as the collet heads 12b are freed from the.