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Originally appearing in Volume V10, Page 468 of the 1911 Encyclopedia Britannica.
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FJG. I.--Flagellata. 1. Chlamydomonas pulvisculus, (Chrysomonadidae). Half of a Ehr. (Chlamydomonadidae) free- large colony, the flagellates em-swimming individual. bedded in a common jelly. a = nucleus. b =contractile vacuole. 0= starch corpuscle. d = cellulose investment. e =stigma (eye-spot). 2. Resting stage of the same, with fourfold division of the cell-contents. Letters as before. 3. Breaking up of the cell-contents into minute biflagellate swarm-spores, which escape, and whose history is not further known. 4 Syncrypta volvox, Ehr. (Chrysomonadidae). A colony enclosed by a common gelatinous test c. a =stigma. b =vacuole(non-contractile). 5. Uroglena volvox, Ehr. 6. Chlorogonium euchlosum, Ehr. (Chlamydomonadidae). a =nucleus. b =contractile vacuole. c =starch grain. d = eye-spot. 7. Chlorogonium euchlorum, Ehr. (Chlamydomonadidae).Copu lation of two liberated microgonidia. a =nucleus. b =contractile vacuole. d =eye-spot (so-called). 8. Colony of Dinobryon sertularia, Ehr. (Chrysomonadidae). 9. Haematococcus palustris, Girod (= Chlamydococcus, Braun, Protococcus, Cohn), one of the (Volvocidae), showing the inter-cellular connective fibrils. a nucleus. b= contractile vacuole. c =starch granule. 19. Two microgametes (spermatozoa) of Volvox globator, L. a= nucleus. b= contractile vacuole. 20. Ripe asexually produced daughter-individual of Volvox minor, Stein, still enclosed in the cyst of the partheno-gonidium. a= young, partheno-gonidia. 21, 22. Trypanosoma sanguinis, Gruby (Haematoflagellates), from the blood of Rana esculenta. a = nucleus; X 500. 23-26. Reproduction of Bodo caudatus, Duj. (Bodonidae), after Dallinger and Drysdale: 23, fusion of several individuals (plasmodium); 24, encysted fusion-product dividing into four; 25, later into eight; 26, cyst filled with swarm-spores. 27. Distigma proteus, Ehbg., O. F. Muller (Euglenidae) ; X 440. Individual with the two flagella, and strongly contracting hinder region of the body. 28. The same devoid of flagella. c, c =the two dark pigment spots (so-called eyes) near the mouth., 29. Oicomonas termo (Monas termo) Ehr. (one of the Oicomonadidae). c = food-ingesting vacuole. d =food-particle; X440. 30. The food-particle d has now been ingested by the vacuole. 31. Oicomonas mutabilis, Kent (Oicomonadidae), with adherent stalk. a = nucleus. b = contractile vacuole. c =food-particle in food vacuole. 32, 33. Cercomonas crassicauda, Duj. (Oicomonadidae), showing two conditions of the pseudopodium-protruding tail. a =nucleus. b =contractile vacuoles. c =mouth. known generally as " plastids " or " chromatophores " impregnated with a lipochrome pigment, whether green (chlorophyll), yellow or brown (diatomin or some allied pigment), or again red (chlorophyll with phycoerythrin). In the active 'condition of such coloured holophytic forms there is usually at least one anterior " eye-spot," of a refractive globule embedded behind in a collection of red pigment granules. The single anterior " flagellum tractellum " of so many of the larger forms acts by the bending over of its free end in consecutive meridians, so as to describe a hollow cone with its apex backwards: we may imitate this by bending the head of a slender sapling round and round while it is implanted in the soil; and the result is to push the water backwards, or in other words to pull the body forwards, the whole rotating on its longitudinal axis as it moves on (Y. Delage). An anterior lateral trailing flagellum may modify this axial rotation, and help in steering. When the animal is at rest—attached by its base or with its body so curved as to resist onward motion—the current produced by the tractellum will bring suspended particles up against the protoplasm at its base of insertion. As noted by E. R. Lankester, the posterior flagellum of many Haemoflagellates, like that of the spermatozoon of Metazoa, propels the cell by a sculling motion behind; he terms it a " pulsellum." Such flagellar motion is distinct from that of cilia, which always move backwards and forwards, with a swift downstroke and a slower recovery in the same plane; though where the flagella are numerous they may behave in this 1 way, and indeed flagella agree with cilia in being mere vibratory extensions of cytoplasm. Symmetrically placed flagella may have a symmetrical reciprocating motion like that of cilia. Many of the Flagellata are parasitic (some haematozoic); the majority live in the midst of putrefying organic matter in sea and fresh waters, but are not known to be active as agents of putrefaction. Dallinger and Drysdale have shown that the spores of Bodo and others will survive an exposure to a higher temperature than do any known Schizomycetes (Bacteria), viz. 2500 to 300° F4hr., for ten. minutes, although the adults are killed at 18o°. The Flagellata are for the most part very minute; the Protomastigopoda rarely exceeding 2oµ in length. The Euglenaceae contain the largest species, up to 130 µ in length, exclusive of the flagellum. Our classification is modified from those of Senn (in Engler and Prantl, Pflanzenfamilien) and Hartog (in Cambridge Natural History). I. RHIZOFLAGELLATA (PANTOSTOMATA) Food taken in by pseudopodia at any part of the body. Order i.—HOLOMASTIGACEAE. Body homaxial with uniform flagella. Multicilia (Cienkowski) ; Grassia (Fisch, in frog's blood and gastric mucus). Order 2.—RHIZOMASTIGACEAE. Flagellum 1, 2 or few, diverging from anterior end. Mastigamoeba (F. E. Schulze). II. EUFLAGELLATA Food taken in at one or more definite mouth-spots, or by a true mouth, or by absorption; or nutrition holophytic. Order i.—PROTOMASTIGACEAE. Contractile vacuole simple, one or more, or absent; either holozoic, ingesting food by a mouth-spot (or 2 or more), saprophytic, or parasitic. Family I.—OICOMONADIDAE. Flagellum 1, sometimes with a tail-like posterior prominence passing into a temporary flagellum, but without other cytoplasmic processes. Oicomonas (Kent) ; Ccrcomonas (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.—CI30AN0FLAGELLIDAE (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 : Protero- spongia (Kent) (fig. 2, 15); Phalansterium (Cienk.) (fig. I, 12), has a slender cylindrical collar, and a branching tubular stalk. FamilY4.—HAEMOFLAGELLIDAE. Formswithacomplexnuclear 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), Her petomonas(Kent),Treponema (Vuillemin) ( = Spirochaete, auctt., nee. Ehrbg.). Family 5.—Am PHIMONADIDAE. Flagella 2 anterior, both directed forward, equal and similar; in stalk sheath, &c., often recalling Choanoflagellata, Amphimon.j (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.—B0DONIDAE. Flagella 2 (or 3) I anterior, the other (i 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, 1o). B. lens is the " hooked " and B. saltans the " springing monad " of Dallinger and Drysdale; Dallingeria (Kent) with a pair of Chrysomonadidae; ordinary individual with widely separated test. a = nucleus. b =contractile vacuole. c =amylon nucleus (pyrenoid). 10. Dividing resting stage of the same, with eight fission pro-ducts in the common test e. i I. A microgonidium of the same. 12. Phalansterium consociatum, Cienk. (Choanoflagellata); X325. Disk-like colony. 13. Euglena viridis, Ehr.; X 300 (Euglenidae). a= pigment spot (stigma). b =clear space. c =paramylum granules. d =chromatophor (endo- chrome plate). 14. Gonium pectorale, O. F. Muller (Volvocineae). Colony seen from the flat side; X 300. a= nucleus. b =contractile vacuole. c =amylon nucleus. 15. Dinobryon sertularia, Ehr. (Chrysomonadidae). a= nucleus. b =contractile vacuole. c =amylon nucleus. d= free colourless flagellates, probably not belonging to Dinobryon. e=stigma (eye-spot). f =chromatophors. 16. Peranema trichophorum, Ehr. (Peranemidae), creeping individual seen from the back; X 140. c =pharynx. d = mouth. 17. Anterior end of Euglena acus, Ehr., in profile. a = mouth. b =vacuoles. c =pharynx. d = stigma (eye-spot). e =paramylum-body. f=chlorophyll corpuscles. 18. Part of the surface of a colony of Volvox globator, L. ing the relation of the individual monads or flagellate zooids to the stem d. 14. Tetramitus rostratus, Perty (Tetramitidae). a = nucleus. b = contractile vacuole. 1, Preterospongia Haeckeli, Saville Kent (Choanoflagellata) ; A social colony of about forty flagellate zooids. a=nucleus. b =contractile vacuole. c =amoebiform cell sunk antero-lateral flagella; Costia necatrix (Leclerq) is also 3-flagellate; causes destructive epidemics in fish-hatcheries. Family 8.—TETRAMITIDAE. Body pyriform, the pointed end posterior; flagella 4 anterior. Tetramitus (Perty) (T. calycinus of Kent, fig. 2, 11, 14), is the " calycine monad " of Dallinger and Drysdale; Trichomonas, Donne, possesses a longitudinal undulating membrane, and is an innocuous human parasite; it is possibly related to H a e mo fl agellates on one hand and to Trichonymphidae on the other. Family 9.—DISTOMATIDAE. Mouth-spots two, or one, with a distinct construction; fla- gella symmetri- cally arranged; nucleus bilobed or geminate. Hexamitus (Duj.) (fig. 2, 5), saprophytic and parasitic ; Trepomonas (Duj.), freshwater; Mega-stoma (Grassi) ( = Lamblia of Blanchard), with con- stricted mouth- spot and blepha- roplast (kinetonucleus) parasitic in the small intestine of Mammals, including Man. Family Io.—TRIcIIo-NYMPHIDAE. Flagella numerous, sometimes accompanied by one or more undulating membranes; cytoplasm highly differentiated; contractile vacuole absent; all parasitic in in-sects (all except Lophomonas in Termites—the so-called White Ants.) Lophomonas(St.) (fig. 2, 9); parasitic in the cock- roach ; Dinenympha (Leidy), Pyrsonympha (Leidy) ; Trichenympha (Leidy) (fig. 3, z). Family II.—OPALINIDAE. Flagella short, numerous, ciliform, uniformly distributed over the flat oval body; nuclei small, numerous, uniform. Only genus, Opalina (Purkinje and Valentin) (fig. 3, 2-6), in bladder and cloaca of the frog (usually regarded as an aberrant ciliate, but E. R. Lankester expressed doubts as to its position in the 9th edition of this encyclopaedia). Order 2.—CHRYSOMONADACEAE. Contractile vacuole simple (in fresh-water forms) or absent; plastids yellow or brown always present; reserves fat. Family I.—CHRYSOMONADIDAE. Body naked, often amoeboid in active state, or sometimes with a cup-like theca, a gelatinous investment, a firm cuticle, or silicified shell; reserves fat or leucosin (starch in Zooxanthella); eye-spot present. Chromulina (Cienk.) often forms a golden scum on tanks; Chrysamoeba (Klebs) ; Hydrurus (Agardh), theca of colony 6, 7. Salpingoeca urceolata, S. Kent (Choanoflagellata) :—6, with collar extended; 7, with collar retracted within the stalked cup. 8. Polytoma uvella, Mull. sp. (Chlamydomonadidae) . 9. Lophomonas blattarum, Stein (Trichonymphidae) from the intestine of Blatta orientalis. so. Bodolens, Mull. (Bodonidae), the wavy filament is a tractellum, the straight one is a trailing thread. II. Tetramitus sulcatus, Stein (Tetramitidae). 12. Anthophysa vegetans, O. F. Muller (Monadidae). A typical, erect, shortly-branching colony stock with four terminal monad-clusters. 13. Monad cluster of the same in optical section, show- 1. Salpingoeca fusiformis, S. Kent (Choanoflagellata). The protoplasmic body is drawn together within the goblet-shaped shell, and divided into numerous spores. 2. Escape of the spores of the same as monoflagellate and swarm-spores. 3. Codosiga umbellata, Tatem (Choanoflagellata) ; adult colony formed by dichotomous growth. 4. A single zooid of the same. a= nucleus. b=contractile vacuole. c=the characteristic " col- lar " of naked stream- ing protoplasm. 5. Hexamita inflata, Duj. (Distomatidae) ; normal adult. 1. Trichonympha agilis, Leidy, from gut of White Ant (Termite). 2. Opalina ranarum, Purkinje parasitic in frog rectum multinucleate adult. 3,4. Binary fissions of same, I-nucleat individual at final stage of fission. 5. Same encysted dejected from rectum to be swallowed by tadpole. 6. Young 1-nucleate individual emerged from cyst, destined to grow, proliferating its nuclei to adult form. a = nucleus. b=food (?) particles in fig. I. within the colonial gelatinous test compared by S. Kent to a mesoderm cell of the sponges. d=similar cell reproducing by transverse fission. e=normal cells, with their collars contracted. f = substance of test. g = individual reproducing by multiple fission, producing microzoospores, comparable to the spermatozoa of sponges. forming branching tubes, simulating a yellow Conferva in mountain torrents; Dinobryon (Ehrb.) (fig. I, 8, i5); Stylochrysalis (St.); Uroglena (Ehrb.); Syncrypta (Ehrb.), and Synura (Ehrb.) (fig. I, 5) form floating spherical colonies; Zooxanthella (randt), symbiotic as ' yellow cells " in Radiolaria Foraminifera, Millepora, and many Actinozoa. 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 fresh-water forms) simple; plastids green, more rarely red, brown or absent; reserves starch; holophy tic or saprophytic. Cryptomonas (Ehrb.); Paramoeba (Greeff) has yellow plastids and shows two cycles, in the one amoeboid, finally encysting to pro-duce 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 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. (I) 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) Chromatephores 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 I.—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. I 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. I, 1, 2, 3); Haematococcus (Agardh) (= Chlamydococcus, A. Braun, Stein) ; Protococcus (Cohn, 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 fresh-water 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. Genera.—Gonium (0. F. Muller) (fig. I, 14); Stephanosphaera (Cohn) ; Pandorina (Bory de Vinc.) ; Eudor'na (Ehr.) ; Volvox (Ehr.) (fig. I, z8, 20). The sexual reproduction of the colonies of the Volvocaceae is one of the most important phenomena presented by the Protozoa. In some families of Flagellata full-grown individuals become amoeboid, fuse, encyst, and then break up into flagellate spores which develop simply to the parental form (fig. I, 23 to 26). In the Chlamydomonadidae a single adult individual by division produces small individuals, so-called ' microgametes." These conjugate with one another or with similar microgametes formed by other adults (as in Chlorogonium, fig. 1, 7); or more rarely in certain genera a microgamete conjugates with an ordinary individual megagamete. The result in either case is a " zygote," a cell formed by fusion of two which divides in the usual way to produce new individuals. The micro-gamete in this case is the male element and equivalent to a spermatozoon; the megagamete is the female and equivalent to an egg-cell. The zygote is a " fertilized egg," or oosperm. In some colony-building forms we find that only certain cells produce by division microgametes; and, regarding the colony as a multicellular individual, we may consider these cells as testis-cells and their micro-gametes as spermatozoa. CYSTOFLAGELLATA(RHYNCHOFLAGELLATAOf E.R.Lankester)and DINOFLAGELLATA are scarcely more than subdivisions of Flagellata; but, following O. Butschli, we describe them separately; the three groups being united into his MASTIGOPHORA. Further Remarks on the Flagellates.—Besides the work of special Protozoologists, such as F. Cienkowski, O. Butschli, F. v. Stein, F. Schaudinn, W. Saville Kent, &c., the Flagellates have been a favourite study with botanists, especially algologists: we may cite N. Pringsheim, F. Cohn, W. C. Williamson, W. Zopf, P. A. Dangeard, G. Klebs, G. Senn, F. Schutt; the reason for this is obvious. They present a wide range of structure, from the simple amoeboid genera to the highly differentiated cells of Euglenaceae, and the complex colonies of Proterospongia and Volvox. By some they are regarded as the parent-group of the whole of the Protozoa—a position which may perhaps better be assigned to the Proteomyxa; but they seem undoubtedly ancestral to Dinoflagellates and to Cystoflagellates, as well as to Sporozoa, and presumably to Infusoria. Moreover, the only distinction between the Chlamydomonadidae and the true green Algae or Chlorophyceae is that when the former divide in the resting condition, or are held together by gelatinization of the older cell-walls (Palmella state), they round off and separate, while the latter divide by a " party wall ' so as to give rise either to a cylindrical filament when the partitions are parallel and the axis of growth constant (Conferva type), or to a plate of tissue when the directions alternate in a plane. The same holds good for the Chrysomonadaceae and Cryptomonadaceae, so that these little groups are included in all text-books of botany. Again among Fungi, the zoospores of the Zoosporous Phycomycetes (Chytrydiaceae, Peronosporaceae, Saprolegniaceae) have the characters of the Bodonidae. Thus in two directions the Flagellates lead up to undoubted Plants. Probably also the Chlamydomonads have an ancestral relation to the Conjugatae in the widest sense, and the Chrysomonadaceae to the Diatomaceae; both groups of obscure affinity, since even the reproductive bodies have no special organs of locomotion. For these reasons the Volvocaceae, Chloromonadaceae, Chrysomonadaceae and Cryptomonadaceae have been united as Phytoflagellates; and the Euglenaceae might well be added to these. It is easy to under-stand the relation of the saprophytic and the holophytic Flagellates to true plants. The capacity to absorb nutritive matter in solution (as contrasted with the ingestion of solid matter) renders the encysted condition compatible with active growth, and what in holozoic forms is a true hypnocyst, a state in which all functions are put to sleep, is here only a rest from active locomotion, nutrition being only limited by the supply of nutritive matter from without, and—in the case of holophytic species—by the illumination: this latter condition naturally limits the possible growth in thickness in holophytes with undifferentiated tissues. The same considerations apply indeed to the larger parasitic organisms among Sporozoa, such as Gregarines and Myxosporidia and Dolichosporidia, which are giants among Protozoa.
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