the pipe having helically extending {TAIL ~ longitudinally spaced corrugations} of {HEAD ~ equal diameter}, convex as viewed from the exterior of the pipe, said corrugations each being arcuate and subtending an angle of less than 180°, 
the pipe having helically extending { TAIL ~ longitudinally spaced corrugations } of { HEAD ~ equal diameter } , convex as viewed from the exterior of the pipe , said corrugations each being arcuate and subtending an angle of less than 180 ° ,
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH