Biological properties of mucus from land snails (Lissachatina fulica) and freshwater snails (Pomacea canaliculata) and histochemical study of mucous cells in their foot oleh Phornphan Phrompanya, Narinnida Suriyaruean, Nattawadee Nantarat, Supap Saenphet, Yingmanee Tragoolpua, Kanokporn Saenphet