They are short and digitiform in A. actinophora, and put forth at one spot or near together; hence the Amoeba has the look of a Cochliopodium. In Trichosphaerium they are elongated, cylindrical, and issue from gaps in the cuticle; in Zonomyxa branched and stretching the cuticle wherever they protrude until it becomes indistinguishable. Amphizonella with a gelatinous envelope retains a more or less globular shape; its motions are very slow; its digitiform pseudopodia perforate the envelope at any point. With a protoplasm of medium density and ectosarc as a well-marked hyaline border, the pseudopodia may have the form of small lobes with hyaline borders, or of conical and pointed ectosarcal processes, occasionally of some length. Locomotion is effected solely by their means or in combination with lobular protrusions. In Amoeba lucida the extremities of the pseudopodia are apt to become spirally twisted. Noticeable peculiarities are, the termination in the Tes-tacean Petalopus of the pseudopodia in plate-like ends, their transformation in the naked Plakopus and Amoeba cellarum into protoplasmic membranes uniting together so as to include funnel-shaped spaces, the villiform protoplasmic processes covering both body and pseudopodia in some specimens of Dactylosphaeriiim vitreum, and the somewhat similar rigid spinules of Deinamoeba (Leidy). The last-named is occasionally invested with a gelatinous coat, beset with minute vertical rods, possibly of Bacterial nature.
1 Nebela (Leidy) is remarkable for the very varied shape of the siliceous elements of its test: they are round or oval discs, or rod-like bodies, varying in length and size, sometimes intermingled, imbedded in a chitinoid membrane (Taranek). They were supposed by Wallich (A. N. H. (3), xiii. PP- 233-4) to be derived possibly from metamorphosed diatom-frustules. The cementing material between the sand-grains, etc. of the test in Difflugia sometimes stains readily. There appears to be a diversity of opinion as to the chitinoid or siliceous nature of the plates in some instances, e. g. in Quadrula.
1Gruber insists strongly on the fact that there is but one kind of'protoplasm in an Amoeba, and that when the granules, etc. either do not follow at once the forward movement of the body, or are confined to a central part, the cause is not the existence of different layers, but the greater density of the protoplasm. See Z. W. Z. xli. pp. 196, 201; Biol. Centralblatt. vi. p. 5.
2Amphitrema stenoslo?na is said by Nusslin to emit both digitiform and filose pseudopodia, one at one end of the test, the other at the other end, simultaneously. The digitiform alone are used in locomotion. Z. W. Z. xl. pp. 718-9.
Very many Nuda possess while in motion, especially flowing motion, a patch of villous processes at the posterior extremity of the body. If the protoplasm is very fluid, the villi are fine and cilia-like, but are seldom seen; if somewhat dense, they are pointed and filamentous; if denser still, stout and blunt. Their significance is unknown. The genus Our-amoeba of Leidy is characterised by having in the same position motionless trailing filaments, cylindrical, tubular, sometimes branched or jointed, aggregated in bundles. It is possible that they are the mycelium of a fungus. Indeed, many Amoebae contain rod-like bodies of varying length, especially aggregated round the nuclei, and the only probable explanation of their nature is that they are symbiotic fungi 1.
The protoplasm may contain colouring matters, sometimes derived from the diatomin or chlorophyl of the food, sometimes intrinsic like the green or yellow globules of Dactylosphaerium, the green, brown, or red granules of Plakopus, the violet granules of Amphizonella violacea, or vacuoles of Zonomyxa, the oil-globule, amber or red in colour, of Diplo-phrys, or the chlorophyl bodies of some species of Difflugia, Hyalosphenia papilio, Heleopera picta, Arcella artocrea, Ditrema flavum, Amphitrema Wrightianum1. Other solid elements are fatty granules, glycogen, and, when chlorophyl is present, starch (?), concretions rounded or crystalline in many Amoebae, probably of an excretory character, mud or sand taken up by Amoebae with fluid protoplasm and by Pelomyxa, elliptical siliceous bodies secreted by the organism in Amoeba granulosa, together with food or its faecal residue. The food, which is either ingested by the pseudo-podia, or by the flowing round it of the protoplasm, consists of Diatoms, algae, dead portions of plants, or minute animals; it is sometimes inclosed in a vacuole, sometimes in direct contact with the protoplasm 2. The latter may be expelled, sometimes inclosed in a vacuole, at any point, but frequently at the posterior end of the moving body in Nuda, or at the mouth of the test in Testacea. Some Amoebae, Arcella and Difflugia proteiformis possess the power of evolving and reabsorbing at will in the protoplasm bubbles of a gas, apparently Carbon dioxide, by means of which they float to the surface of the waters they inhabit.
Non-contractile vacuoles are present in numbers in some Amoebae, in Pelomyxa, etc. Occasionally they have been observed to disappear slowly, others appearing at another spot as in Zonomyxa. Contractile vacuoles may be entirely absent, as in Pelomyxa and some others. Their number when present is liable to variation. Amoebae very generally have 1-3; many Testacea a single large one; others several, in the posterior part of the body, which travel to the mouth of the test to burst. Arcella may have as many as twelve in a ring round the body.
1Korotneff states that the appendages of his Longicauda, = Ouramoeba, amoebina, undergo changes tinder unfavourable circumstances, the contents of the filaments becoming segmented into small squarish highly refractile bodies (? spores); A. Z. Expt. viii. 1879-80, p. 472. In Leidy's 0. vorax the filaments are said to be cylindrical tubes, in 0. botulicauda they consist of a series of elliptical joints, 1-4, adapted end to end. Leidy says they resemble mycelial filaments; see his PI. ix. in the Fresh-water Rhizopoda, etc. Gruber thinks they are fungoid, and mentions that he has seen the fungoid filaments of Amoeba binucleata protruded in bundles under the action of chromic acid; Z. W. Z. xli. p. 211.