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