The primary criterion used to classify organisms into different biological kingdoms is the cell type. This classification is based on the structural and functional characteristics of cells, which are the basic building blocks of all living organisms. Cells can be broadly classified into two types: prokaryotic and eukaryotic.

Prokaryotic Cells:
Prokaryotic cells are simple, primitive cells that lack a distinct nucleus and membrane-bound organelles. They are found in organisms belonging to the kingdom Monera, which includes bacteria and cyanobacteria (also known as blue-green algae). Prokaryotes are single-celled organisms that have a relatively simple structure and are capable of diverse metabolic activities.

Eukaryotic Cells:
Eukaryotic cells are more complex cells that have a distinct nucleus and membrane-bound organelles. They are found in organisms belonging to the remaining four kingdoms: Protista, Fungi, Plantae, and Animalia.

- Protista: The kingdom Protista consists of eukaryotic organisms that are mostly unicellular, although some may be colonial or multicellular. These organisms are primarily aquatic and include protozoans, algae, and slime molds.

- Fungi: The kingdom Fungi comprises eukaryotic organisms that are mostly multicellular, although some may be unicellular. Fungi obtain their nutrition by decomposing organic matter, and they include molds, yeasts, and mushrooms.

- Plantae: The kingdom Plantae consists of eukaryotic organisms that are multicellular and primarily photosynthetic. Plants have specialized structures for photosynthesis, such as chloroplasts, and they include trees, flowers, and grasses.

- Animalia: The kingdom Animalia comprises eukaryotic organisms that are multicellular and heterotrophic. Animals have complex organ systems and exhibit a wide range of behaviors. This kingdom includes mammals, birds, reptiles, amphibians, and fish.

In summary, the primary criterion used to classify organisms into different biological kingdoms is the type of cells they possess. Prokaryotic cells are found in the kingdom Monera, while eukaryotic cells are found in the kingdoms Protista, Fungi, Plantae, and Animalia. This classification is based on the structural and functional characteristics of cells, which determine the overall organization and behavior of organisms.