Rhizopod Protozoa with lobose, digitiform or filose pseudopodia, sometimes branched, rarely anastomosing, either locomotor and alimentative in ftmction or the latter alone, sometimes perhaps tactile. The presence of a hyaline border ('ectosarc') depends on the density of the protoplasm. The body is either naked, enveloped in a complete gelatinoid or chitinoid coat, or lodged in a monothalamous test with one or two apertures, in composition either gelatinoid, chitinoid, encrusted with foreign bodies, or made up of chitinoid or siliceous plates cemented together. Non-contractile vacuoles are sometimes present in numbers; contractile vacuoles, sometimes absent, are usually single, or limited to a few. There may be one, two, or many nuclei. Freshwater, terricolous, rarely marine, saprophytic, or parasitic.
It has been shown that in the naked Amoebina with dense protoplasm, a superficial thin layer undergoes coagulation by contact with the water in which the animal lives, but that it is continually dissolved in locomotion and as continually formed anew. The degree to which it is specialised is variable in different species; when more resistent than usual it causes puckers and lines on the surface, e. g. in Amoeba verrucosa, and conular eminences at the bases of the pseudopodia, e.g. A. tentaculata. In a few instances however a permanently differentiated layer or cuticle is met with. Zonomyxa has a chitinoid (?) resistent membrane which is stretched out to invisible thinness where the protoplasm is in motion or protruded as pseudopodia; Amphizonella violacea is clothed with a gelatinous coat pierced by the pseudopodia; Trichosphaerium, with an investment of short spines set on a thin membrane, rounded gaps here and there giving exit to the pseudopodia. The marine Amoeba obtecta inhabits a cup-shaped gelatinous theca. The test of those Amoebina which possess one is always monothalamous, when chitinoid either colourless, yellow or brown, according to thickness. It has a single aperture, except in three genera, Diplophrys, Ditrema and Amphitrema, with two at opposite poles.
Pores at the base of the test for the escape of water have been observed in two instances, in Hyalosphenia papilio and Nebela bursella. The test is cup-shaped in Cochliopodium, more or less hemispherical, with the aperture limited by an inwardly turned rim in Arcella, usually globular, oval or pyriform. Owing to compression, or to obliquity of the mouth, it may become bilateral; in Difflugia spiralis and Pleurophrys (=Pseudodif-flugia ?) Helix it is usually said to form a half-coil. As to the former, however, Leidy states that it is retort-shaped, and an internal septum stretched across the neck of the retort gives the appearance of a spiral twist. The simple chitinoid shell may be very thin, flexible and closely applied to the body (Pamphagus, Diplophrys)), thicker, but not rigid (Cochliopodium), stiff (Ditrema), and the body not in complete contact with it (Hyalosphenia, Platoum), as is nearly invariably the case in the genera to be mentioned. In Quadnda it consists of transverse rows of squarish chitinoid plates, and in Arcella of two membranes, an inner thin, homogeneous, and an outer composed of hexagonal prisms, placed vertically, and filled with water.
Difflugia, Amphitrema, etc, have foreign bodies of very various kinds (diatom-frustules, sand-grains, etc.) attached to the outer surface of the chitinoid membrane, or held together by a cement, either gelatinous or siliceous (?). In Euglypha and its allies the plates composing the test appear to be silicified; they are probably held together by a chitinoid cement. The plates of the row surrounding the mouth are pointed and serrated, and that aperture has consequently a margin strongly denticulated in Etiglypha\ it is beaded in Trinema, but as a rule it is smooth. The remaining plates of the test are oval, or hexagonal, disposed in regular longitudinal lines and sometimes overlapping, exceedingly minute in Cypkoderia, and implanted in a chitinoid membrane1. The test is sometimes furnished with processes, or tubercles, with a crest or with spines, coarse or fine, and in Placocista moveable.
The protoplasm varies much in density in the naked Amoebina or Nuda. When it is fluid the granules, etc., which it contains are distributed throughout its whole substance, and it may or may not show a hyaline edge at the spot which is extending onwards in locomotion. But with an increase in density, the granules, etc, are confined to the more central portion and leave a more or less pronounced hyaline border. Hence a distinction into an outer ectosarc and an inner endosarc l. In the Testacea the granules are frequently aggregated in a middle zone. Their protoplasm also, when the test is rigid, does not under ordinary circumstances fill it, but a connection is maintained between the two by contractile threads which retract the animal when it is molested. The configuration of the pseudopodia appears to depend on the density of the protoplasm. In the Testacea they are emitted only at the aperture or in amphistomatous genera at both apertures, from a mass of clearer protoplasm, and they are either few and digitiform, occasionally containing granules and sometimes slightly branched, e. g. in Arcella, Difflugia, etc, or they are filose, i. e. filamentous, and generally somewhat numerous, simple or branched, rarely widely extended as in Pamphagus, and seldom undergoing anastomosis2. In the Nuda they are subject to much variation.
When the protoplasm is fluid it not infrequently flows as a whole in one direction; or the pseudopodia are broad, more or less irregular lobes, seldom, as in Amoeba Proteus, digitiform, and the granules and other contents of the protoplasm flow into them. A steady onward flow is also observable in the locomotion of species with very dense protoplasm and a well-wdefined ectosarc, e. g. Hyalodiscus with the ectosarc as a border on the advancing side; Amoeba verrucosa and A. terricola with an uneven advancing edge or knob-like projections and wrinkled surface. So too in A. tentaculata and A. actinophora with coagulated surface, in Zonomyxa and Tricho-sphaerium with differentiated cuticular structures (supra). But the four last-named have also special pseudopodia, purely ectosarcal except in Zonomyxa, of no use however as organs of locomotion, but serving solely as nutritional, and perhaps also as tactile organs. In A. tentaculata they protrude from the ends of conular eminences, are short, pointed, and display curving motions; they are retracted in locomotion or persist tentaclelike on the advancing margin.