There are zoas, palys and protopalys. All three different but similar. Some palys are smaller than some zoas. Some have very long stalks, some are short. So size not 'always' a determining factor, just sometimes. Palys are usually bigger though, yes. The real answer is palys use sand etc to build tissue. It is all 'scientificy' and I am tired. I will look for the link though that explains it better. I forgot if it is palys or protopalys that use the grains....I will go look.