Diatoms have a cell wall that includes silica, while dinoflagellates have a cell wall that includes cellulose.
Explanation:
Diatoms are unicellular algae. Some of them can live in colonies forming filaments or ribbons, they are surrounded by a cell wall made of silica called a frustula.
They are microscopic animals, almost always single-celled. They have flagella. They can be divided into two large groups differentiated by the presence or absence of cellulosic plaques in the cell wall or amphysm.
The "frustule" cell wall of diatoms. A dinoflagellate isn't constructed this way. The frustule is composed of two parts which are pretty much identical except for size -- one of the parts is slightly smaller than the other.