KEY TO MARINE QENERA OF CHOANOFLAGELLIDA. 1. Without gelatinous 3 or membranous test 2. With gelatinous 4 or membranous test 3. a. Attached forms: 1. Without a stalk, or with Genus *_Monosiga_ a very short one 2. With a long, simple, stalk Genus *_Codonosiga_ 3. With a long, branched, stalk Genus _Codonocladium_ b. Free-swimming Genus _Desmarella_ 4. Colonial, and with a gelatinous Genus _Proterospongia_ covering * Presence at Woods Hole indicated by asterisk. KEY TO FAMILIES AND MARINE GENERA OF HETEROMASTIGIDA. 1. Two flagella nearly equal in size Family _Bodonidæ_ One main and 2 accessory flagella Family _Trimastigidæ_ Family _Bodonidæ_: 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 KEY TO FAMILIES AND MARINE GENERA OF EUGLENIDA. 1. With deeply-insunk pharynx; 2 no mouth With pharynx and distinct mouth Family _Peranemidæ_ 2. Body plastic; usually with Family _Euglenidæ_ chromatophores and eye-spot Body plastic; no chromatophores; Family _Astastidæ_ no eye-spot Family _Euglenidæ_: Body _Euglena_-like, inclosed Genus _Trachelomonas_ in shell with round opening for exit of flagellum Family _Astastidæ_: Body with one flagellum Genus *_Astasia_ Family _Peranemidæ_: 1. Body striped; plastic; Genus _Heteronema_ two diverse flagella 2. Body striped; not plastic; Genus *_Anisonema_ posterior flagellum longer than the other 3. Body striped; not plastic; Genus _Entosiphon_ with rod-like organ in pharynx * Presence at Woods Hole indicated by asterisk. Genus MASTIGAMOEBA F. E. Schultze '75. (Kent '81; Bütschli '86; Klebs '92; Senn 1900.)