The Protozoa are the very lowest forms of animal existence, the beginning and dawn of living things. They first appear as minute shapeless particles of semi-fluid sarcode moving on the surface of the waters. The pseudopodia, or false feet, with which they move, are merely lobes of their own substance which they project and retract. In creatures of a somewhat higher grade the form is definite, the pseudopodia, numerous and filamental, serving for locomotion and catching prey; and from the resemblance they bear to the slender roots of plants are called Rhizopods.[[3]] The microscopic organisms possessing these means of locomotion and supply, are of incalculable multitudes, and of innumerable forms. Thus the waters, as of old, still ‘bring forth abundantly the moving creature that hath life;’ in them the lowest types of the two great kingdoms have their origin, yet they are diverse in the manifestation of the living principle, that slender but decided line which separates the vegetable from the animal Amœba.
Class I.—Rhizopoda.
The Amœba, which is the simplest of the group, is merely a mass of semi-fluid jelly, ‘changing itself into a greater variety of forms than the fabled Proteus, laying hold of its food without members, swallowing it without a mouth, digesting it without a stomach, appropriating its nutritious material without absorbent vessels or a circulating system, moving from place to place without muscles, feeling (if it has any power to do so) without nerves, multiplying itself without eggs, and not only this, but in many instances forming shelly coverings of a symmetry and complexity not surpassed by those of any testaceous animal.’
Fig. 86. Amœba princeps.
Such is the description given by Dr. Carpenter of the Amœba and its allies. The Amœba princeps, which is the type of the naked group, [fig. 86], is merely a shapeless mass of semi-fluid sarcode, coated by a soft, pellucid and highly contractile film, called diaphane by Mr. W. J. Carter, and in many forms of Amœba the whole is inclosed in a transparent covering. It is in the interior semi-fluid sarcode alone, that the coloured and granular particles are diffused, on which the hue and opacity of the body depend, for the ectosarc or external coat is transparent as glass. These creatures, which vary in size from the 1⁄2800 to the 1⁄70 of an inch in diameter, are found in the sea, but chiefly in ponds inhabited by fresh-water plants. They move irregularly over the surface of the water, slowly and continually changing their form by stretching out portions of their gelatinous mass in blunt finger-like extensions, and then drawing the rest of it into them; thus causing the whole mass to change its place. Before it protrudes these pseudopodia or false feet, there is a rush of the internal semi-fluid matter to the spot, due to the highly contractile power of the diaphane, which is often so thin and transparent as to be scarcely perceptible.
When the creature in its progress meets with a particle of food, it spreads itself over it, draws it into its mass, within which a temporary hollow or vacuole is made for its reception; there it is digested, the refuse is squeezed out through the external surface; the nutritious liquid that is left in the vacuole seems to be dispersed in the sarcode, for the vacuole disappears. An Amœba often spreads itself over a Diatom, draws it into a vacuole newly made to receive and digest it; the siliceous shells of the diatom are pushed towards the exterior, and are ultimately thrust out; then the vacuole disappears, either immediately or soon after. These improvised stomachs are the earliest form of a digestive system.
Besides the vacuoles of which there may be several at a time, the slow and nearly rhythmical pulsations of a vesicle containing a subtle fluid may be seen, which changes its position in the interior of the sarcode with every motion of the Amœba. It gradually increases in size, then diminishes to a point, and as some of the digestive vacuoles nearest the surface of the animal are observed to undergo distension when the vesicle contracts, and to empty themselves gradually as it fills, Dr. Carpenter thinks it can hardly be doubted that the function of the vesicle is to maintain a continual movement of nutritious matter, among a system of channels and vacuoles excavated in the substance of the body. It is the first obscure rudiment of a circulating system.
In all the Amœbæ the semi-fluid sarcode, with the numerous bodies suspended in it, rotates at a varied rate within the pellucid coat; a motion presumed to be for respiration, that is to exchange carbonic acid gas for oxygen, so indispensable for animal life.[[4]]
Although like other animals, the Amœba cannot change inorganic into organic matter, as the vegetable Amœba can do, these two Protozoa are similar in one mode of reproduction; for portions of the animal Amœba or even one of the pseudopodia separate from the gelatinous mass, move to a little distance on the surface of the water, and become independent Amœbæ.