Biology and Its Makers / With Portraits and Other Illustrations

by William A. Locy

The Discovery of the Simplest Animals and the Progress of Observations upon Them

These single-celled animals, since 1845 called protozoa, have become of unusual interest to biologists, because in them the processes of life are reduced to their simplest expression. The vital activities taking place in the bodies of higher animals are too complicated and too intricately mixed to admit of clear analysis, and, long ago, physiologists learned that the quest for explanations of living activities lay along the line of investigating them in their most rudimentary expression. The practical recognition of this is seen in our recent text-books upon human physiology, which commonly begin with discussions of the life of these simplest organisms. That greatest of all text-books on general physiology, written by Max Verworn, is devoted largely to experimental studies upon these simple organisms as containing the key to the similar activities (carried on in a higher degree) in higher animals. This group of animals is so important as a field of experimental observation that a brief account of their discovery and the progress of knowledge in reference to them will be in place in this chapter.

Discovery of the Protozoa.—Leeuwenhoek left so little unnoticed in the microscopic world that we are prepared to find that he made the first recorded observations upon these animalcula. His earliest observations were communicated by letter to the Royal Society of London, and were published in their Transactions in 1677. It is very interesting to read his descriptions expressed in the archaic language of the time. The following quotation from a Dutch letter turned into English will suffice to give the flavor of his writing:

"In the year 1675 I discovered living creatures in rainwater which had stood but four days in a new earthen pot, glazed blew within. This invited me to view the water with great attention, especially those little animals appearing to me ten thousand times less than those represented by Mons. Swammerdam, and by him called water-fleas or water-lice, which may be perceived in the water with the naked eye. The first sorte by me discovered in the said water, I divers times observed to consist of five, six, seven or eight clear globules, without being able to discover any film that held them together or contained them. When these animalcula, or living atoms, did move they put forth two little horns, continually moving themselves; the place between these two horns was flat, though the rest of the body was roundish, sharpening a little towards the end, where they had a tayle, near four times the length of the whole body, of the thickness (by my microscope) of a spider's web; at the end of which appeared a globule, of the bigness of one of those which made up the body; which tayle I could not perceive even in very clear water to be mov'd by them. These little creatures, if they chanced to light upon the least filament or string, or other such particle, of which there are many in the water, especially after it has stood some days, they stook entangled therein, extending their body in a long round, and striving to dis-entangle their tayle; whereby it came to pass, that their whole body lept back towards the globule of the tayle, which then rolled together serpent-like, and after the manner of copper or iron wire, that having been wound around a stick, and unwound again, retains those windings and turnings," etc.[2]

Any one who has examined under the microscope the well-known bell-animalcule will recognize in this first description of it, the stalk, and its form after contraction under the designation of a 'tayle which retains those windings and turnings.'

There are many other descriptions, but the one given is typical of the others. He found the little animals in water, in infusions of pepper, and other vegetable substances, and on that account they came soon to be designated infusoria. His observations were not at first accompanied by sketches, but in 1711 he sent some drawings with further descriptions.

O. Fr. Müller.—These animalcula became favorite objects of microscopic study. Descriptions began to accumulate and drawings to be made until it became evident that there were many different kinds. It was, however, more than one hundred years after their discovery by Leeuwenhoek that the first standard work devoted exclusively to these animalcula was published. This treatise by O. Fr. Müller was published in 1786 under the title of Animalcula Infusoria. The circumstance that this volume of quarto size had 367 pages of description with 50 plates of sketches will give some indication of the number of protozoa known at that time.

Ehrenberg.—Observations in this domain kept accumulating, but the next publication necessary to mention is that of Ehrenberg (1795-1876). This scientific traveler and eminent observer was the author of several works. He was one of the early observers of nerve fibres and of many other structures of the animal frame. His book of the protozoa is a beautifully illustrated monograph consisting of 532 pages of letterpress and 69 plates of folio size. It was published in 1836 under the German title of Die Infusionsthierchen als Vollkommene Organismen, or "The Infusoria as Perfect Organisms." The animalcula which he so faithfully represented in his sketches have the habit, when feeding, of taking into the body collections of food-particles, aggregated into spherical globules or food vacuoles. These are distinctly separated, and slowly circulate around the single-celled body while they are undergoing digestion. In a fully fed animal these food-vacuoles occupy different positions, and are enclosed in globular spaces in the protoplasm, an adjustment that gave Ehrenberg the notion that the animals possessed many stomachs. Accordingly he gave to them the name "Polygastrica," and assigned to them a much higher grade of organization than they really possess. These conclusions, based on the general arrangement of food globules, seem very curious to us to-day. His publication was almost simultaneous with the announcement of the cell-theory (1838-39), the acceptance of which was destined to overthrow his conception of the protozoa, and to make it clear that tissues and organs can belong only to multicellular organisms.

Ehrenberg (Fig. 31) was a man of great scientific attainments, and notwithstanding the grotesqueness of some of his conclusions, was held in high esteem as a scientific investigator. His observations were accurate, and the beautiful figures with which his work on the protozoa is embellished were executed with such fidelity regarding fine points of microscopic detail that they are of value to-day.

Dujardin, whom we shall soon come to know as the discoverer of protoplasm, successfully combated the conclusions of Ehrenberg regarding the organization of the protozoa. For a time the great German scientist tried to maintain his point, that the infusoria have many stomachs, but this was completely swept away, and finally the contention of Von Siebold was adopted to the effect that these animals are each composed of a single cell.