traordinary variety and complexity first fascinated 

 and then overawed tlie earlier microscopists. Though 

 some workers delineated the protozoans in superb 

 engravings which we still admire, they also put curi- 

 ous interpretations upon what they saw, because 

 they tried to find stomachs and intestines and kid- 

 neys in little animals that they visualized always in 

 terms of vertebrate anatomy. Only when it was re- 

 alized that protozoans are not miniatures of the 

 larger beasts but animals organized in a very dilTer- 

 ent way from all the other groups did biologists begin 

 to make real headway. Now we understand the 

 protozoan body to consist of a minute bit of a com- 

 plex mixture of substances known as protoplasm, 

 bounded externally by a membrane and containing 

 at least one formed body, the nucleus. In all other 

 animals the body is built up of a very large number 

 of such nucleated units of protoplasm, called cells. 

 Whether to think of a protozoan as a single cell, or 

 to consider it noncellular (or acellular) because 

 the body is not partitioned up into units as in the 

 many-celled metazoan groups, is a matter still de- 

 bated by specialists. For our purpose it is enough to 

 keep in mind that a protozoan is not comparable to 

 a single cell of a man but to his whole body. 



Size is not the criterion for putting protozoans 

 into a special subkingdom Protozoa apart from all 

 the animals of the other subkingdoms. As we shall 

 see later, there are groups of metazoans that are en- 

 tirely microscopic, and some as big as lobsters that 

 have free-swimming microscopic stages when they 

 first hatch from the egg. The larger protozoans regu- 

 larly capture and devour adult metazoans related to 

 the lobster, though sometimes not without a truly 

 heroic struggle. The important distinction, as has 

 already been pointed out, is that of body design. And 

 it is at least as remarkable that protozoans are able 

 to carry on all the complex processes of life within a 

 single microscopic globule as that many-celled ani- 

 mals can do the same thing through the combined 

 activities of vast numbers of walled-off and special- 

 ized units. 



Having just settled the protozoans comfortably in 

 their place, it seems a little belated to point out that 

 some zoologists have tossed them out of the animal 

 kingdom altogether. The problem of their status be- 

 gan to puzzle microscopists from the moment that 

 Leeuwenhoek first saw green globules swimming un- 

 der their own power. Today many zoologists still 

 maintain that green unicellular forms that move 

 about actively are properly members of the irritable, 

 restless animal kingdom. Equally firm are the bota- 

 nists who claim that such forms belong to the plant 

 kingdom, since the green color is that of chlorophyll. 



Those who feel less sure about what to do with 

 sedentary one-celled plants that have actively swim- 

 ming sex cells, or where to place swimming green 



forms that can lose their pigment and feed like any 

 animal when conditions change, decline to take sides 

 in this tug of war. They prefer to set up a third king- 

 dom of living organisms, the Protista, that admits 

 any form not divided into separate cells. Colonial 

 forms are included because they do not show enough 

 division of labor among the aggregated cells to be 

 considered truly multicellular. 



The very existence of modern plant-animals that 

 are green but swim actively and that can shift from 

 plantlike to animal-like feeding habits suggests that 

 there was at one time a primitive stock of green mo- 

 tile organisms, perhaps very much like modern green 

 flagellates (p. 22) from which both plant and ani- 

 mal kingdoms have arisen. Because this book is about 

 animals, it is more convenient here to put flagellated 

 organisms that swim about actively, whether green or 

 colorless, in the phylum Protozoa. This first great 

 grouping in the animal kingdom, as mentioned ear- 

 lier, is set aside in a special subkingdom of its own. 



Numbers 



The protozoans or "first animals" deserve their 

 name in more than just the chronological sense. Ev- 

 ery larger animal that we carefully examine turns 

 out to harbor one or more species of protozoans, and 

 protozoans themselves may play host to even smaller 

 uninvited protozoans. So it is quite safe to guess that 

 the number of individual protozoans in the world 

 exceeds by far that of all other animal species com- 

 bined. In the seas, which cover three-quarters of the 

 globe, free-living protozoans occur from top to bot- 

 tom. The billions upon billions of protozoans in such 

 masses of water are really incomprehensible to our 

 simple mammalian minds. 



Despite the inconceivable numbers of protozoans 

 in all bodies of water and in all surface soils, and the 

 intimate association enjoyed by some twenty-five 

 different species of protozoans that live in man, no 

 large grouping of animals is so unfamiliar at first 

 hand to all but professional biologists. In the nine- 

 teenth century in England and on the Continent, 

 every gentleman of wealth who had any pretensions 

 to intellectual curiosity displayed a microscope in his 

 living room and perhaps belonged to a microscope 

 club in which he could exchange his latest observa- 

 tions with like-minded friends. This has gone out of 

 fashion, and the only microscopes found in most 

 homes are toylike versions for children. The almost 

 incredible fairyland of beautiful and bizarre crea- 

 tures that swim, feed, pursue each other, and re- 

 produce — unabashed by the gaze of anyone who 

 chooses to look at them through a microscope — may 

 some day again become a source of entertainment 

 and intellectual satisfaction after we have pushed 

 most of the bigger animals to near extinction. For 

 protozoans are accessible to anyone who can spare 



[17 



