Family 2.—Coccolithophoridae. Body invested in a spherical test strengthened by calcareous elements, tangential circular plates, “coccoliths,” “discoliths,” “cyatholiths,” or radiating rods “rhabdoliths.” These are often found in Foraminiferal ooze and its fossil condition, chalk; when coherent as in the complete test, they are known as “coccospheres” and “rhabdospheres.” Coccolithophora (Lohmann), Rhabdosphaera (Haeckel).
Order 3.—CRYPTOMONADACEAE. Contractile vacuole (in freshwater forms) simple; plastids green, more rarely red, brown or absent; reserves starch; holophytic or saprophytic. Cryptomonas (Ehrb.); Paramoeba (Greeff) has yellow plastids and shows two cycles, in the one amoeboid, finally encysting to produce a brood of flagellulae; in the other flagellate, and multiplying by longitudinal fission (it differs from Mastigamoeba in possessing no flagellum in the amoeboid state, though it takes in food amoeba-fashion); Chilomonas (Ehrb.).
Order 4.—CHLOROMONADACEAE. Contractile vacuoles 1-3, a complex of variable arrangement; pellicle delicate; plastids discoid chlorophyll-bodies; reserves oil; eye-spot absent even in active state; holophytic or saprophytic, though with an anterior blind tubular depression simulating a pharynx. Coelomonas (St.), Vacuolaria (Cienk.).
Order 5.—EUGLENACEAE. Vacuole large, a reservoir for one or more accessory vacuoles, contractile and opening to the surface by a canal (“pharynx”) in which are planted one or two strong flagella; pellicle strong often striated; nucleus large, chromatophores green, complex or absent; reserves paramylum granules of definite shape, and oil; nutrition variable; body stiff or “metabolic,” never amoeboid. Among the true Flagellates these are the largest, few being below 40 μ and several attaining 130 μ in length of cell-body (excluding flagellum). Encysted condition common; the green forms sometimes multiply in this state and simulate unicellular Algae.
Family 1.—Euglenidae. Radial (monaxial) forms; nutrition saprophytic or holophytic, mostly one flagellate. (1) Chromatophore large; eye-spot conspicuous. Euglena (Ehrb.) (Fig. 1, 13, 17), with flexible cuticle and metabolic movements (this is probably Priestley’s “green matter” through which he obtained oxygen gas)—a very common genus; Colacium (Ehbg.), in its resting state epizoic on Copepoda, which it colours green; Eutreptia (Perty), biflagellate; Ascoglena (St.); Trachelomonas (Ehrb.), with a hard brown cuticle; Phacus (Nitszche), with a firm rigid pellicle, often symmetrically flattened; Cryptoglena (Ehbg.). (2) Chromatophores absent. Astasia (Duj.), body metabolic; Menoidium (Perty), body not metabolic, somewhat inflected and crescentic; Sphenomonas (Stein), with a short accessory trailing flagellum in front peeled; Distigma (Ehbg.) (Fig. 1, 27, 28), very metabolic, with two unequal flagella and two dark pigment spots.
Family 2.—Peranemidae. Bilaterally symmetrical, often creeping, pharynx highly developed, with a firm rod-like skeleton, sometimes protrusible; nutrition saprophytic and holozoic. Peranema (Ehbg.) and Urceolus (Mereschowsky), uni-flagellate creeping, very metabolic. Petalomonas (St.), uni-flagellate flattened with a deep ventral groove, not metabolic; Heteronema (Duj.) and Tropidoscyphus (St.), with a small accessory anterior trailing flagellum; Anisonema (Duj.) and Entosiphon (St.), with the trailing flagellum as long as the tractellum or even much longer.
Order 6.—VOLVOCACEAE. Contractile vacuole simple anterior; cell always enclosed in a cellulose wall (sometimes gelatinous) perforated by the two (more rarely four, five) diverging anterior flagella; reserves starch; chlorophyll almost always present, except in Polytoma, sometimes masked by a red pigment; nutrition usually holophytic, rarely saprophytic, never holozoic. Brood-division in active state common, radial.
Family 1.—Chlamydomonadidae. Cell-wall firm not gelatinous, rarely forming colonies. Fore-end of the body with two or four (seldom five) flagella. Almost always green in consequence of the presence of a very large single chromatophore. Generally a delicate shell-like envelope of membranous consistence. 1 to 2 simple contractile vacuoles at the base of the flagella. Usually one eye-speck. Division of the protoplasm within the envelope may produce four, eight or more new individuals. This may occur in the swimming or in a resting stage. Also by more continuous fission microgametes of various sizes are formed. Conjugation is frequent.
Genera.—Chlorangium (Stein), lacking green chlorophyll; Chlorogonium (Ehr.) (Fig. 1, 6, 7); Polytoma (Ehr.) (Fig. 2, 8); Chlamydomonas (Ehr.) (Fig. 1, 1, 2, 3); Haematococcus (Agardh) (= Chlamydococcus, A. Braun, Stein); Protococcus (Conn, Huxley and Martin); Chlamydomonas (Cienkowski), causes red snow and “bloody rain”; Carteria (Diesing), quadri-flagellate; Spondytomorum (Ehrb.), forming floating colonies; Coccomonas (St.); Phacotus (Perty); Zoochlorella (Brandt), is the name given to undetermined Chlamydomonads found multiplying in the resting state within and in symbiotic relation to other Protozoa, to the freshwater sponge, Ephydatia, Hydra viridis, and to the Turbellarian, Convoluta viridis (in which last species the active form has been recognized as a Carteria).
Family 2.—Volvocidae. Cell-wall gelatinous; always associated in colonies; cells, as in Family 1. The number of individuals united to form a colony varies very much, as does the shape of the colony. Reproduction by the continuous division of all or of only certain individuals of the colony, resulting in the production of a daughter colony (from each such individual). In some, probably in all, at certain times copulation of the individuals of distinct sexual colonies takes place, without or with a differentiation of the colonies and of the copulating cells as male and female. The result of the copulation is a resting zygospore (also called zygote or oospermo or fertilized egg), which after a time develops itself into one or more new colonies.