1. Body very plastic, Genus *_Bodo_
almost amoeboid

Body not plastic; with large Genus *_Oxyrrhis_
anterior cavity, holding flagella

Family _Trimastigidæ_:

1. With an undulatory membrane Genus _Trimastix_
between accessory flagella

Without such membrane; flagella Genus _Costia_
contained in a ventral groove
while at rest

* Presence at Woods Hole indicated by asterisk.


KEY TO MARINE GENERA OF POLYMASTIGIDA.

1. Body flattened; ends rounded; Genus _Trepomonas_
sides hollowed; often with
wing-like processes; cross
section S-shaped

2. Body pyriform; one large Genus _Tetramitus_
asymmetrical groove; 4 flagella

3. Body spherical; many flagella Genus _Multicilia_
equally distributed