Answer:
This is because fluids have a polar nature, so they couldn´t cross the lipidic bilayer that constitutes the plasma membrane
Explanation:
Plasma membranes are contituted by lipid bilayers. These kind of compounds are hidrophobic (they reject water) and don´t permit the freely passage of hidriphilic molecules that can disolve in water, constituing fluids. Because of that cells developed transport mechanism like endocytosis.
Stem cells are what allow planaria to grow and regenerate as the stem cells can change into any of the cells that make up the planaria.
