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