When the organism has been starved for a time the medulla becomes nearly hyaline: when full-fed, on the contrary, perfectly opaque with granules.
Reproduction takes place by the transverse division or fission of the organism into two new individuals. The process is repeated for some time, the resulting organisms gradually diminishing in size. Conjugation then takes place. Two individuals fuse temporarily by their oral regions: and during fusion there is a division of both nucleus and paranucleus (supra), and apparently an interchange of protoplasm. When the conjugating individuals separate, 'rejuvenescence" sets in, i.e. the nuclei and paranuclei are reconstituted, and each individual regains the full dimensions of the species. Multiplication by fission then re-commences as before.
Amoeba Proteus or A. princeps, the Proteus animalcule (Fig. 13 C), agrees with Paramecium in being a unicellular organism: it differs from it in the absence of a permanently fixed cortex, and in the character of its organs of locomotion, which are non-vibratile lobes of the protoplasm known as pseudopodia (C: p. and p'.). It is to be found in the upper layers of soft ooze at the bottom of still clear lakes, ponds, and ditches, or creeping on the under surface of the fronds of Duckweed and the floating leaves of various aquatic plants.
The body consists of a pale, jelly-like, finely granular protoplasm, capable of both extension and contraction at the will of the animal. The outer layer of protoplasm retains a clear aspect and firm nature and constitutes the ectosarc (C: e.): but it forms no definite cuticle, though it is possible that the surface in contact with the water coagulates into a pellicle more resistent than the underlying strata. The interior portion, the endo-sarc (C: e'.), is fluent and watery, and by contrast more or less opaque, owing to admixture with various elements partly formed by the chemical activity of the protoplasm itself, partly taken up by it from without The substances of the former class, according to Leidy, who has carefully studied this organism, consist of (1) minute granules, pale or dark: (2) spherical corpuscles of largish size, homogeneous character, and either colourless or feebly yellow, liquid or semiliquid: (3) round or oval bodies resembling starch grains both in appearance and chemical reaction: (4) colourless, yellow, or brown oval globules, with dark border, highly refractile and apparently fatty in nature (C:/.): (5) minute crystals and (6) water drops, colourless or feebly yellow.
Substances taken up from without are (1) food balls, soft, generally spherical and uniform in size but very variable in colour and composition, according to the nature of the organism (Rotifer, Infuso-rian, etc.) from which they are derived: (2) food materials of a firmer nature such as Diatoms, Desmids, Unicellular Algae, which retain their shape owing to the presence of a skeleton or firm cuticle: (3) various foreign bodies, organic and inorganic, picked up from the surface on which the animal happens to be creeping. The food materials are frequently but not invariably inclosed in a vacuole. They as well as other substances can be taken up at any point in the body, either by the union of two inclosing pseudopodia or by the protoplasm flowing over and around them. Expulsion of faecal and other materials as a rule takes place at the posterior extremity (C:f.) when the animal is in motion. 'The discharge is rather sudden and is often accompanied with the escape of some viscid fluid' (Leidy). The rift in the protoplasm closes at once, leaving no trace of its existence.
It is a noteworthy fact that, in spite of the soft nature of the inclosing protoplasm, the food-balls, vacuoles contractile and non-contractile, retain their shape and integrity in every movement of the endosarc and animal.
There is always one nucleus, and sometimes more than one (C: n.), and a contractile vacuole. The position of the nucleus varies, but it is generally somewhere about or behind the centre of the body. It is colourless, homogeneous, finely or coarsely but uniformly granular: in shape a compressed sphere or disc with convex, flat, or concave surfaces and rounded edge. The contractile vacuole (C:c.v.) is a clear globe usually placed behind the nucleus at the posterior extremity. It enlarges slowly and collapses suddenly. A new vacuole is formed at or near the spot occupied by its predecessor. There is rarely more than one of these pulsatile spaces present.
1 The distinction of the protoplasm into ecto- and endo-sarc is probably, strictly speaking, accidental; and there is nothing to prove that the ectosarc cannot and does not mix with the endosarc. In other words, the distinction is not to be regarded as permanent, in the same sense that the distinction between cortex and medulla in Paramecium is permanent.
The animal when in a state of repose or after being disturbed forms a spherical or oval ball, about 1/125 of an inch in size. It sometimes occurs in this condition surrounded by a delicate membrane forming a 'hypnocyst.' It is then 'resting' owing to drought or plentiful nutrition. When it passes into a state of active motion, the surface of the body is covered with numerous clear protrusions of ectosarc. Certain of these elongate into pseudopodia while the remainder are withdrawn, and at the same time the animal begins to flatten out. The endosarc flows into the growing pseudopodia, which at first extend in various and opposite directions. But sooner or later one or two elongate in a given direction, and the rest retract, and the animal moves in a determinate course. It is now, according to the phase of shape assumed by the pseudopodia, either ramose, as in Fig. 13, C, dendroid, or palmate. When it floats freely suspended in water the pseudopodia extend in all directions, giving it a radiate or stellate appearance. The pseudopodia attain in their growth characters which are distinctive of the species or the group to which these Rhizopodan organisms belong. In Amoeba Proteus they are finger-like, i.e. digitate, simple (C:p.) or branched (C: p'.). Their tips are either blunt or tapering.
Elongation takes place by an onward flow of ectosarc followed by the extrusion into the ectosarc of a current of endosarc. The flow is, as a rule, not even but more or less sudden, as though there were a surface-resistance to be overcome. The posterior part of the body, where the protoplasm is receding, has often a lobed or mulberry-like appearance, as shown in Fig. 13, C. Similar lobes may be observed on a pseudopodium during its retraction. Any single lobe in the posterior mass might commence to elongate, and grow into a pseudopodium: and the direction of the animal's movement, indicated in C. by the arrows, might be thus reversed. In the extended condition this species may measure the 1/50 of an inch.
An Amoeba has been observed dividing into two. The protoplasmic bridge or filament connecting the two halves, gradually becomes more and more delicate and lengthened, and then finally snaps. It is uncertain whether or no Amoeba Proteus ever forms spores in the shape of minute Amoeba or Amoebulae, such as have been observed in Pelomyxa palustris.
Protozoa, Ray Lankester, Encyclopaedia Brit. (ed. ix.) xix. 1885.
Infusoria (= Ciliata\ Saville Kent, Manual of the Infusoria, 2 vols., and Plates, London, 1880-82. For Parameciwn, see.vol. ii. p. 483; PL xxvi. Figs. 28-30.
Amoeba. Leidy, Fresh-water Rhizopoda of North America, United States Geological Survey, xii. 1879. For A. Proteus, see p. 31; Pis. i; ii; iv, Fig. 25; vii, Figs. 13-19; viii, Figs. 17-30. Cf. Gruber, Z.W.Z. xli. 1885.