Family 1.—Oicomonadidae. Flagellum 1, sometimes with a tail-like posterior prominence passing into a temporary flagellum, but without other cytoplasmic processes. Oicomonas (Kent); Cercomonas (Dujardin) (Fig. 1, 32, 33); Codonoeca (James-Clark), with a gelatinous theca.
Family 2.—Bicoecidae. Differs from Oicomonadidae in a unilateral proboscidiform process next the flagellum; often thecate and stalked, forming branched colonies, like Choanoflagellates in habit. Bicoeca (J.-Cl.), Poteriodendron.
Family 3.—Choanoflagellidae (Choanoflagellata, Kent; Craspedomonadina, Stein). As in previous families, but with flagellum surrounded by an obconical or cylindrical rim of cytoplasm, at the base of which is the ingestive area. The cells of this group have the morphology of the flagellate cells (choanocytes) of sponges. They are often colonial, and in the gelatinous colony of Proterospongia, the more internal cells (Fig. 2, 15) pass into a definite “reproductive state.” Many stalked forms are epizoic on Entomostracan Crustacea.
(a) Naked forms often stalked: Monosiga (Kent), stalked solitary; Codosiga (Kent) (Fig. 2, 3), stalked social; Desmarella (Kent), unstalked, and Astrosiga (Kent), stalked, form floating colonies.
(b) Forms enclosed in a vase-like shell: Salpingoeca (J.-Cl.); (Fig. 2, 1, 6, 7) recalling the habit of Monosiga and Cod siga; Polyoeca forming a branched free swimming colony.
(c) Forms surrounded by a gelatinous sheath: Proterospongia (Kent) (Fig. 2, 15); Phalansterium (Cienk.) (Fig. 1, 12), has a slender cylindrical collar, and a branching tubular stalk.
Family 4.—Haemoflagellidae. Forms with a complex nuclear apparatus, and a muscular undulating membrane with which one or two flagella are connected, parasitic in Metazoa (often in the blood). Trypanosoma (Gruby) (Fig. 1, 21, 22), Herpetomonas(Kent), Treponema (Vuillemin)(= Spirochaete, auctt., nec. Ehrbg.).
Family 5.—Amphimonadidae. Flagella 2 anterior, both directed forward, equal and similar; in stalk sheath, &c., often recalling Choanoflagellata, Amphimonas (Kent), Diplomitus (Kent); Spongomonas (St.), with thick branching gelatinous sheath.
Family 6.—Monadidae. Flagella 2 (3), anterior all directed forwards, one long the other (or 2) accessory, short.
Monas (St.); Anthophysa (Bory) (Fig. 2, 12, 13), with the stalk composed of the accumulation of faeces at the hinder end of the cells of the colony.
Family 7.—Bodonidae. Flagella 2 (or 3) 1 anterior, the other (1 or 2) antero-lateral and trailing or becoming fixed at the end to form a temporary anchor.
Bodo (Ehrb.) (figs. 1, 23-26 and 2, 10). B. lens is the “hooked” and B. saltans the “springing monad” of Dallinger and Drysdale; Dallingeria (Kent) with a pair of antero-lateral flagella; Costia necatrix (Leclerq) is also 3-flagellate; causes destructive epidemics in fish-hatcheries.