Classification.—The classification of the Protozoa is no easy task. We omit here, for obvious reasons, the unmistakable Plant Protists that have a holophytic or saprophytic nutrition; that are, with the exception of a short period of locomotion in the young reproductive cells, permanently surrounded with a wall of cellulose or fungus-cellulose, and that multiply and grow freely in this encysted state: to these consequently we relegate the Chytridieae, which so closely allied to the Proteomyxa and the Phycomycetous Fungi; and the Confervaceae, which in the resting state form tubular or flattened aggregates and are allied to the green Flagellates; besides the Schizophyta. At the opposite pole stand the Infusoria in the strict sense, with the most highly differentiated organisation found in our group, culminating in the possession of a nuclear apparatus with nuclei of two kinds, and exhibiting a peculiar form of conjugation, in which the nuclear apparatus is reorganised. The Sporozoa are clearly marked off as parasites in living animals, which mostly begin life as sickle-shaped cells and have always at least two alternating modes of brood-formation, the first giving rise to aplanospores, wherein is formed the second brood of sickle-shaped, wriggling zoospores. The remainder, comprising the Sarcodina, or Rhizopoda in the old wide sense (including all that move by pseudopodia during the great part of their active life), and the Flagellata in the widest sense, are not easy to split up; for many of the former have flagellate reproductive cells, and many of the latter can emit pseudopodia with or without the simultaneous retraction of their flagella. The Radiolaria are well defined by the presence in the body plasm of a central capsule marking it off into a central and a peripheral portion, the former containing the nucleus, the latter emitting the pseudopodia. Again, on the other hand, we find that we can separate as Flagellata in the strict sense the not very natural assemblage of those Protista that have flagella as their principle organs of movement or nutrition during the greater part of their active life. The remaining groups (which with the Radiolaria compose the Sarcodina of Bütschli), are the most difficult to treat. The Rhizopoda, as we shall limit them, are naked or possess a simple shell, never of calcium carbonate, have pseudopodia that never radiate abundantly nor branch freely, nor coalesce to form plasmatic networks, nor possess an axial rod of firmer substance. The Foraminifera have a shell, usually of calcium carbonate, their pseudopodia are freely reticulated, at least towards the base; and (with the exception of a few simple forms) all are marine. The Mycetozoa are clearly united by their tendency to aggregate more or less completely into complex resting-groups (fructifications), and by reproducing by unicellular zoospores, flagellate or amoeboid, which are not known to pair. The Heliozoa resemble the Radiolaria in their fine radiating pseudopodia, but have an axial filament always present in each, and lack the central capsule; and are, for the most part, fresh-water forms. Finally, the Proteomyxa forms a sort of lumber-room for beings which are intermediate between the Heliozoa, Rhizopoda, and Flagellata, usually passing through an amoeboid stage, and for the most part reproducing by brood-formation. Zoospores that possess flagella are certainly known to occur in some forms of Foraminifera, Rhizopoda, Heliozoa, and Radiolaria, though not by any means in all of each group.[[63]]
| A. Pseudopodia the principal means of locomotion and feeding; flagella absent or transitory | I. Sarcodina |
| (1) Plastogamy only leading to an increase in size, never to the formation of "fructifications." | |
| (a) Pseudopodia never freely coalescing into a network nor fine to the base | Rhizopoda. |
| (*) Ectoplasm clear, free from granules; pseudopodia, usually blunt | Rhizopoda Lobosa |
| (**) Ectoplasm finely granular; pseudopodia slender, branching, but not forming a network, passing into the body by basal dilatation | Rhizopoda Filosa |
| (b) Pseudopodia branching freely and coalescing to form networks; ectoplasm granular; test usually calcareous or sandy | Foraminifera |
| (c) Pseudopodia fine to the very base; radiating, rarely coalescing. | |
| (i.) Pseudopodia with a central filament | Heliozoa |
| (ii.) Pseudopodia without a central filament. | |
| (*) Body divided into a central and a peripheral part by a "central capsule" | Radiolaria |
| (**) Body without a central capsule | Proteomyxa |
| (2) Cells aggregating or fusing into plasmodia before forming a complex "fructification" | Mycetozoa |
| B. Cells usually moving by "euglenoid" wriggling or by excretion of a trail of viscid matter; reproduction by alternating modes of brood-formation, rarely by Spencerian fission | II. Sporozoa |
| C. Flagella (rarely numerous) the chief or only means of motion and feeding | III. Flagellata |
| D. Cilia the chief organs of motion, in the young state at least; nuclei of two kinds | IV. Infusoria |
CHAPTER III
PROTOZOA (CONTINUED): SARCODINA
I. Sarcodina.
Protozoa performing most of their life-processes by pseudopodia; nucleus frequently giving off fragments (chromidia) which may play a part in nuclear reconstitution on division; sometimes with brood-cells, which may be at first flagellate; but never reproducing in the flagellate state.[[64]]
1. Rhizopoda
Sarcodina of simple form, whose pseudopodia never coalesce into networks (1),[[65]] nor contain an axial filament (2), which commonly multiply by binary fission (3), though a brood-formation may occur; which may temporarily aggregate, or undergo temporary or permanent plastogamic union, but never to form large plasmodia or complex fructifications as a prelude to spore-formation (4); test when present gelatinous, chitinous, sandy, or siliceous, simple and 1-chambered (5).
Classification.[[66]]
| I. Ectoplasm distinct, clear; pseudopodia blunt or tapering, but not branching at the apex | Lobosa |
| Amoeba, Auctt.; Pelomyxa, Greeff; Trichosphaerium, A. Schneid.; Dinamoeba, Leidy; Amphizonella, Greeff; Centropyxis, Stein; Arcella, Ehr.; Difflugia, Leclercq; Lecqueureusia, Schlumberger; Hyalosphenia, Stein; Quadrula, F. E. Sch.; Heleopera, Leidy; Podostoma, Cl. and L.; Arcuothrix, Hallez. | |
| II. Ectoplasm undifferentiated, containing moving granules; pseudopodia branching freely towards the tips | Filosa |
| Euglypha, Duj.; Paulinella, Lauterb.; Cyphoderia, Schlumb.; Campascus, Leidy; Chlamydophrys, Cienk.; Gromia, Duj. = Hyalopus, M. Sch. | |