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Updated: May 6, 2025
Every living object is an association, a collectivity. The atom does not exist alone, as an individual; the atom is invisible and impalpable and it does not live. And the complexity of the aggregation, the federation of the parts constantly increases with the ascent in the zoological series from protozoa to Man.
The Gastraea itself has originated from the simple multicellular vesicle, the Blastaea, and this in turn must have been evolved from the lowest circle of unicellular animals, to which we give the name of Protozoa. We have already considered the most important primitive type of these, the unicellular Amoeba, which is extremely instructive when compared with the human ovum.
Thus the sponges are later than the protozoa, the corals succeed the sponges, the sea-urchins come after the corals, the shell-fish follow the sea-urchins, the articulates are later than the shell-fish, the vertebrates are later than the articulates.
Adjacent to the physicist's corner of the hut Atkinson is quietly pursuing the subject of parasites. Already he is in a new world. The laying out of the fish trap was his action and the catches are his field of labour. Constantly he comes to ask if I would like to see some new form and I am taken to see some protozoa or ascidian isolated on the slide plate of his microscope.
So again, the Infusoria possess all the characters of animality, but it can hardly be said that they are as clearly allied to the worms as they are to the Noctilucae. On the whole, it appears to me to be most convenient to adhere to the old plan of calling such of these low forms as are more animal in habit, Protozoa, and such as are more vegetal, Protophyta.
The trypanosomes are fighting with the normal blood cells. Here we have the lowest group of animal life, the protozoa, at work killing the highest, man." Kennedy needed nothing more than the breathless stillness to convince him of the effectiveness of his method of presenting his case. "Now," he resumed, "let us leave this blood-sucking, vampirish tse-tse fly for the moment.
It has descended from the Cyclostoma by a profound degeneration, and these in turn from the fishes; even the Ascidia and the whole of the Tunicates are merely degenerate fishes! Following out this curious theory, Dohrn came to contest the general belief that the Coelenterata and Worms are "lower animals"; he even declared that the unicellular Protozoa were degenerate Coelenterata.
In other chapters, particularly the one dealing with the house-fly and typhoid, we shall see how it is that insects are often important factors in spreading some of the most dreaded of the bacterial diseases. The Protozoa, or one-celled animals, belonged to an unknown world before the invention of the microscope.
I do not think we could ask nature for more complete proof that human beings have evolved from one-cell ancestors as simple as modern protozoa beyond the obvious facts of human transformation during development.
The PROTOZOA. The Protozoa are represented throughout the whole range of geological series, from the Lower Silurian formation to the present day.
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