Each tube has a slot extending longitudinally at least along {HEAD ~ a part} of {TAIL ~ its length} 
Each tube has a slot extending longitudinally at least along { HEAD ~ a part } of { TAIL ~ its length }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL