Why protist are better termed as Acellular?

Viruses, viroids and prions are included in acellular microorganisms. These organisms when present outside the living host cell are considered non-living, as they behave as inert particles, but inside the host cell they can replicate and behave like living organisms. They use host machinery to synthesis all their components as well as genome replication and assemble into hundreds of new particles.

Acellular slime molds might also be called plasmodial slime molds. Plasmodial slime molds are actually one cell. They can be huge, a couple of feet across, but still only one cell. They are able to grow so large because the one cell is multinucleated. That word means the cell has several nuclei inside.

Protists lack cellular organisation and are not limited by cytoplasmic membrane hence they are referred to as acellular