circular aperture 18 through which a weft yarn 20 is passed and leave an exit slot 22 defined between respective leading ends of the parts 14 and 16 the arcuate part 16 is provided near the leading end thereof with sealing means 24 which is constructed of a resilient material and projects toward the exit slot 22 as shown as will be well stated in the following description the sealing means 24 functions to almost close the exit slot 22 while the weft yarn is travelling through the aperture 18 and to open the exit slot 22 upon exodus of the weft yarn 20 from the aperture 18 and furthermore to restrict the leakage of the pressurized air from the aperture 18 during the weft yarn penetration as seen in the drawings the sealing means 24 substantially consists of a flat spring portion 24a extending from an inner surface of the arcuate part 16 and a thin screen or web portion 24b arranged to cover the clearance no numeral defined by the flat spring portion 24a in an undeflected state and the inner surface of the leading end of the arcuate part 16 a leading end 25 of the flat spring portion 24a is bent radially outwardly with respect to the aperture 18 of the tooth.