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Updated: May 6, 2025
Other protozoa differ in possessing constant forms, or in having constant vibratile processes, or shells of some kind, while in still other cases like individuals combine to make colonies which are more or less definite and permanent. Here at the very foot of the organic scale are found animals which seem to be entirely different from those above.
He tells us that large numbers of the lower animals have no nervous system, though they exercise the functions of sensation and will like the higher animals. In the unicellular Protozoa, which do not form germinal layers, there is, of course, neither nervous system nor skin. But in the second division of the animal kingdom also, the Metazoa, there is at first no nervous system.
All of which comes perilously near to being a paradox, and commends itself to the attention of the S. P. C. A. One of the transatlantic liners dumped out at Ellis Island a lump of protozoa which was expected to evolve into an American citizen.
The various examples we have mentioned and the groups to which they belong clearly occupy different places in the scale which begins with the protozoa and extends upward to the most complicated and differentiated animals.
The Chromacea build up their plasm directly from inorganic matter; the Bacteria feed on organic matter. Hence, if we logically divide the protist kingdom into plasma-forming Protophyta and plasma-consuming Protozoa, we must class the Bacteria with the latter; it is quite illogical to describe them as is still often done as Schizomycetes, and class them with the true fungi.
The protozoa include also the flagellata, a great, very poorly defined mass of forms occupying the boundary between the plant and animal kingdoms. They are usually unicellular, and their protoplasm is surrounded by a thin, structureless membrane. This prevents their putting out pseudopodia as organs of motion.
It may therefore be useful to consider one or two intermediate forms and the parallel embryonic stages of higher animals, and to see how the higher many-celled animal originates from the unicellular stage. The amoeba is an illustration of a great kingdom of similar, practically unicellular forms, which have played no unimportant part in the geological history of the globe. These are the protozoa.
To explain this Weismann went back to the unicellular protozoa. These animals are undoubtedly influenced by environment and vary under its stimuli. Here the variations were stamped upon the germ-plasm, and the commingling of these variously stamped germ-plasms has resulted in all the variations of higher animals. Of late Weismann has modified and greatly improved this portion of his theory.
I may mention that on some of the hottest days of last summer I made some experiments, when the temperature both of the laboratory and inside the cylinder was 75 deg.F. I used test tubes containing boiled potatoes instead of meat, and found that the tube in the air, after 48 hours, abounded not simply with bacteria and other small bodies present in decomposition, but with the large and varied forms of protozoa, while the tube inside the cylinder contained no signs of decomposition whatever.
All forms are in it, from the mightiest mammals to the protozoa which the microscope suspects rather than surely discovers. Every time molecule touches molecule in the depths, a new spark of tiny life must flare up, else never so many could inhabit the water. The coarser aggregations of these we see in bewildering profusion and variety every time the tides fall back and leave the rocks bare.
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