Members of this family have 4 anterior flagella, an axostyle and a single nucleus. They apparently lack a parabasal body. The only genus found in domestic animals is Monocercomonoides.
Members of this genus have 4 anterior flagella in 2 pairs, a pelta and an axostyle which is generally filamentous. Nie (1950) described 1 to 4 strand-like funises which stain with protargol in 4 species of this genus from the guinea pig. The funis is a costa-like structure extending backwards just beneath the body surface. Members of this genus occur in insects, amphibia, reptiles and a number of mammals. They are non-pathogenic.
Monocercomonoides caprae (Das Gupta, 1935) (syn., Monocercomonas caprae) was described from the rumen of the goat in Indiae The body is ovoid, 6 to 12 u long and 4 to 8 u wide.
Monocercomonoides caviae (Cunha and Muniz, 1921) Nie, 1950, M. quadrifunilis Nie, 1950, M. wenrichi Nie, 1950 and M. exilis Nie, 1950 occur in the cecum of the guinea pig.
Monocercomonoides sp. was found by Saxe (1954) in the laboratory rat and golden hamster. He transmitted it from the hamster to the rat. This species awaits morphologic study and specific characterization.