Not all protistan cells have a ventral side. When people refer to ventral, they will probably be guided by one ofthese criteria:

- The ventral is that face of the cell at which a non-apical mouth opens (see Amoeba, Anisonema, Actinomonas, Paramecium, Lacrymaria, Didinium)

- The ventral is that face of the cell that is in contact with the substrate when the cell is moving (see Actinomonas, Crinolina, Telonema)

- If there is a polar mouth, the front is that part of the cell that has the mouth (see Lacrymaria, Didinium, Telonema)

Arcella, a testate amoeba, has an aperture through which the pseudopodia emerge. This defines the ventral side.

Peranema, a euglenid, is seen in side view crawling over the surface of a bubble. One long flagellum extends in front of the cell. A second flagellum adheres to the cell body (not visible here) and it is this face of the cell that is in contact with the substrate. It is the ventral side.

Stylonychia, a hypotrich ciliate, with clustrers of cilia forming cirri that are used to 'walk' over the ground. These cirri define the ventral side.

Chilodonella, a hypostome ciliate with a mouth on the ventral side.

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