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