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