This family is transitional between the Mastigasida and the Sarcodasida. Both amoeboid and flagellate stages occur in its life cycle.
The flagellate stage has 2 flagella. The amoeboid stage has lobopodia and resembles Vahlkampfia. The nucleus is vesicular, with a large endosome. The contractile vacuole is conspicuous. The cysts are uninucleate. Naegleria lives on bacteria and is free-living in stagnant water or coprophilic.
Naegleria gruberi (Schardinger) (syn., Dimastigamoeba gruberi) is found in stagnant water and is also coprophilic. The active amoebae are 10 to 36 by 8 to 18 u and have a single vesicular nucleus 3 to 4 u in diameter. The nucleus has a central endosome and sparse granules of peripheral chromatin. The flagellate stage is 18 by 9 u, ovoid, and has 2 equal anterior flagella. It can be produced from the amoeboid stage by flooding the culture with distilled water and exposing it to air. The cysts are spherical, 8 to 12 u in diameter, translucent, with a single nucleus and several large spherical chromatoid bodies when first formed. The cyst wall is double, and the outer wall is perforated by 3 to 8 pores.
The nucleus is vesicular, with a large endosome. The flagellate stage has 3 flagella (4 according to Bovee, 1959) and the amoeboid stage is relatively small. The cysts are uninucleate, with a smooth wall.
Trimastigamoeba philippinensis Whitmore, 1911 was first found in human feces. Bovee (1959) rediscovered it in sewage-seepage into a spring in Florida and redescribed it. According to Whitmore (1911), the flagellate stage has 3 (occasionally 2 or 4) anterior flagella and measures 16 to 22 by 6 to 8 u. Bovee, however, found that there are actually 4 anterior flagella which arise in pairs from 2 basal granules adjacent to the nucleus. The flagella lie in an anterior, gullet-like, cylindrical invagination and extend 20 to 25 u, beyond it. According to Bovee, the fully formed flagellate stage is 17 to 20 u long, its larger rear end is 6.5 to 7.5 u in diameter, the narrower anterior end is 4.5 to 5.5 u in diameter and the anterior pocket is 7 u deep.
The amoeboid stage was said by Whitmore (1911) to be 16 to 18 u in diameter. Bovee (1959) said that it is 12 to 18 u in diameter when at rest and 30 to 40 u long and 14 to 20 u wide during rapid locomotion. It moves quickly by means of rapidly-extruded eruptive waves at its frontal margin. It feeds principally on bacteria and has a contractile vacuole which is formed by the union of several small vacuoles in about 2 minutes. The cysts are oval to subspherical. Whitmore (1911) gave their dimensions as 13 to 14 by 8 to 12 u.