A. Fission usually longitudinal (transverse only in a cyst), or if multiple, radial and complete: pellicle absent, thin, or if armour-like, with not more than two valves.
I. Food taken in at any part of the body by pseudopodia 1. PANTOSTOMATA
Multicilia Cienk.; Mastigamoeba F. E. Sch. (Fig. 37, 4).

II. Food taken in at a definite point or points, or by absorption, or nutrition holophytic.
1. No reticulate siliceous shell. Diameter under 500 µ (1⁄50").
* Contractile vacuole simple (one or more).
(α) Colourless: reserves usually fat: holozoic, saprophytic or parasitic 2. Protomastigaceae
(β) Plastids yellow or brown: reserves fat or proteid: nutrition variable: body naked, often amoeboid in active state (C. nudae), or with a test, sometimes containing calcareous discs ("coccoliths," "rhabdoliths") of peculiar form (C. loricatae) 3. Chrysomonadaceae
Chromulina Cienk.; Chrysamoeba Klebs; Hydrurus Ag. Dinobryon Ehrb. (Fig. 37, 11); Syncrypta Ehrb. (Fig. 37, 12); Zooxanthella Brandt; Pontosphaera Lohm.; Coccolithophora Lohm.; Rhabdosphaera Haeck.
(γ) Green, (more rarely yellow or brown) or colourless: reserves starch: fission longitudinal 4. Cryptomonadaceae
Cryptomonas Ehrb. (Fig. 37, 9); Paramoeba Greeff.
(δ) Green (rarely colourless): fission multiple, radial 5. Volvocaceae
** System of contractile vacuoles complex, with accessory formative vacuoles or reservoir, or both.
(ε) Pellicle delicate or absent: pseudopodia often emitted: excretory pore distinct from flagellar pit: reserves fat 6. Chloromonadaceae
Chloramoeba Lagerheim; Thaumatomastix, Lauterborn.
(ζ) Pellicle dense, tough or hard, often wrinkled or striate: contractile vacuole discharging by the flagellar pit. Nutrition variable 7. Euglenaceae
Euglena Ehrb.; Astasia Duj. (Fig. 37, 3); Anisonema Duj.; Eutreptia Perty (Fig. 42, p. [124]); Trachelomonas Ehrb. (Fig. 37, 1); Cryptoglena Ehrb.
2. Skeleton an open network of hollow siliceous spicules. Plastids yellow. Diameter under 500 µ. 8. Silicoflagellata
Dictyocha Ehrb.
3. Diameter over 500 µ. Mouth opening into a large reticulate endoplasm: flagella 1, or 2, very unequal. 9. Cystoflagellata
Noctiluca Suriray (Fig. 48); Leptodiscus R. Hertw.
B. Fission oblique or transverse: flagella two, dissimilar, the one coiled round the base of the other or in a traverse groove; pellicle often dense, of numerous armour-like plates 10. Dinoflagellata
Ceratium Schrank; Gymnodinium Stein; Peridinium Ehrb. (Fig. 46); Pouchetia Schütt; Pyrocystis Murray (Fig. 47); Polykrikos Bütschli.

The Protomastigaceae and Volvocaceae are so extensive as to require further subdivision.

Protomastigaceae

I. Oral spots 2. Flagella distant in pairs. Distomatidae
II. Oral spot 1 or 0.

A. Flagellum 1.
(a) No anterior process: often parasitic Oikomonadidae
Oikomonas K. (Figs. 37, 2, 8); Trypanosoma Gruby (Fig. 39, a-f); Treponema Vuill. (Fig. 39, g-i).
(b) Anterior process unilateral or proboscidiform: cell often thecate Bicoecidae
Bicoeca Clark; Poteriodendron St.
(c) Anterior process a funnel, surrounding the base of the flagellum: cells often thecate.
(i.) Funnel free Craspedomonadidae
Codosiga Clark; Monosiga Cl.; Polyoeca Kent; Proterospongia Kent; Salpingoeca Cl.
(ii.) Funnel not emerging from the general gelatinous investment Phalansteridae
B. Flagella 2, unequal or dissimilar in function, the one sometimes short and thick.
(a) Both flagella directed forwards Monadidae
Monas St.; Anthophysa Bory (Fig. 37, 13).
(b) One flagellum, usually the longer, turned backwards Bodonidae
Bodo St. (Fig. 38).
C. Flagella 2, equal and similar Amphimonadidae
Amphimonas Duj.; Diplomita K. (Fig. 37, 10); Rhipidodendron St. (Fig. 37, 14).
D. Flagella 3 Trimastigidae
Dallingeria K. (Fig. 37, 6); Costia Leclercq.
E. Flagella 4 or more: mostly parasitic in Metazoa Polymastigidae
Trichomonas Donne; Tetramitus Perty (Fig. 37, 7); Hexamitus Duj.; Lamblia Blanchard.
F. Flagella numerous, sometimes constituting a complete ciliiform investment, and occasionally accompanied by an undulating membrane: parasitic in Metazoa.
(a) Flagella long: nucleus single: parasitic in insects Trichonymphidae
Dinenympha Leidy; Joenia Grassi; Pyrsonympha Leidy; Trichonympha Leidy; Lophomonas St.; Maupasia Schew.
(b) Flagella short, ciliiform, uniformly distributed: nuclei very numerous, all similar: parasitic in Amphibia Opalinidae
Opalina Purkinje and Valentin (Fig. 41).

Volvocaceae

A. Cells usually isolated, separating after fission or brood-formation. Usually green (sometimes red), more rarely colourless saprophytes Chlamydomonadidae
Chlamydomonas Ehrb.; Phacotus Perty; Polytoma Ehrb.; Sphaerella Sommerf. (Fig. 43); Zoochlorella.
B. Cells multiplying in the active state by radial divisions in the same plane and usually incurving to form a spherical colony, united in a gelatinous investment, sometimes traversed by plasmic threads Volvocidae
Gonium O.F.M.; Eudorina Ehrb.; Pandorina Bory (Fig. 45); Stephanosphaera Cohn; Volvox L. (Fig. 44).

Fig. 37.—Various forms of Flagellata. 2, 6-8, 10, 13, 14, Protomastigaceae; 11, 12, Chrysomonadaceae; 9, Cryptomonadaceae; 1, 3, Euglenaceae; 4, Pantostomata: note branched stalk in 13; branched tubular theca in 14; distinct thecae in 11; stalk and theca in 10. In 2, flagellate (a) and amoeboid (b) phases are shown; in 5, flagellate (a) and Heliozoan (b) phases[[116]]; in 8 are shown two stages in the ingestion of a food particle (f); chr, plastoids; c.vac, contractile vacuole; f, food particle; g, gullet; l, theca; nu, nucleus; p, protoplasm; per, peristome; v.i, vacuole of ingestion. (From Parker and Haswell, mostly from Bütschli's Protozoa.)

The modes of nutrition are threefold: the simplest forms live in liquids containing decaying organic matter which they absorb through their surface ("saprophytic"): others take in food either Amoeba fashion, or into a vacuole formed for the purpose, or into a definite mouth ("holozoic"): others again have coloured plastids, green or brown or yellow ("holophytic"), having the plant's faculty of manufacturing their own food-supply. But we meet with species that show chromatophores at one time and lack them at another; or, again, the same individual (Euglena) may pass from holozoic life to saprophytic (Paramoeba, some Dinoflagellates) as conditions alter.

Many secrete a stalk at the hinder end: by "continuous" formation of this, without rupture at fission, a branching colony is formed (Polyoeca). This stalk may have a varying consistency. In Anthophysa (Fig. 37, 13) it appears to be due to the welding of excrementitious particles voided at the hinder end of the body with a gelatinous excretion; but the division of the stalk is here occasional or intermittent, so that the cells are found in tufts at the apex of the branches. A corresponding secretion, gelatinous or chitinous, around the body of the cell forms a cup or "theca," within which the cell lies quite free or sticking to it by its surface, or attached to it by a rigid or contractile thread. The theca, again, may assume the form of a mere gelatinous mass in which the cell-bodies may be completely plunged, so that only the flagella protrude, as in Volvocidae, Proterospongia (Fig. 75, p. [182]), and Rhipidodendron (Fig. 37, 14). Often this jelly assumes the form of a fan (Phalansterium), the branching tubes of which it is composed lying for some way alongside, and ultimately diverging. In Hydrurus, the branching jelly assumes the form of a branching Confervoid.[[117]]