The path through {HEAD ~ the shell wall} may release a substance contained in {TAIL ~ the shell} or allow a substance to enter the shell 
The path through { HEAD ~ the shell wall } may release a substance contained in { TAIL ~ the shell } or allow a substance to enter the shell
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH