Phylum Protozoa is classified based on

The correct answer is locomotory organ.

Phylum Protozoa includes a diverse group of single-celled eukaryotic microorganisms. The traditional classification of Protozoa into different subphyla or classes is primarily based on their mode of locomotion and the types of locomotory organelles they possess.

The main groups classified under Protozoa based on their locomotory structures are:
1. Mastigophora (Flagellates): These protozoans move using one or more whip-like structures called flagella (e.g., Euglena, Trypanosoma).
2. Sarcodina (Amoeboids): These organisms move and capture food by projecting temporary extensions of the cytoplasm called pseudopodia, or false feet (e.g., Amoeba).
3. Ciliophora (Ciliates): These protozoans possess numerous short, hair-like projections called cilia that beat coordinately for movement and feeding (e.g., Paramecium).
4. Sporozoa: These are parasitic protozoans that lack specialized locomotory organelles in their adult stage and typically reproduce by forming spores (e.g., Plasmodium).

Therefore, the locomotory organ is the primary criterion used for the classification of Phylum Protozoa.