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