426 EVOLUTION OF ANIMALS Part V 



reach them. They became, and still are, successful parasites of the human 

 digestive tract and, with 'f't help of man's insect associates, have been widely 

 introduced into human blood. 



Compared with multicellular animals, protozoans are only relatively simple; 

 many are extraordinarily complex. No near kin of the ameba has come to f!y 

 like a bird, but neither can birds make a living on bacteria, as many protozoans 

 do. The multicellular animals created opportunities for the protozoans; they 

 have never displaced them. 



Characteristics. Protozoans are minute unicellular animals that carry on all 

 the fundamental processes of the life of higher animals. They live in all sorts 

 of places and in different ways with one limitation, that for at least part of 

 their life span their surroundings must be wet, actually a limitation of every 

 animal. 



Most protozoans have animal-like structures, flagella, cilia, and special 

 openings for the entrance of food; some have light receptors containing the 

 visual pigment carotene probably present in all types of eyes; others have 

 neuromotor fibrils suggestive of nervous systems; many bear paralyzing trich- 

 ocysts that are shot out in defense or attack (Fig. 21.1). Likewise, most pro- 

 tozoans are animal-like in their activities, such as the digestion of food and 

 elimination of water, the conjugation or mating of Paramecium, and the 

 fiercely carnivorous behavior of Didinium. Contrasting with these are the 

 plant-like flagellates that are green with chlorophyll and contain cellulose, 

 such as the green spheres of Volvox, and the myriad euglenas that give a 

 pasture pond the look of a spring greensward. Euglenas carry on photosyn- 

 thesis as truly as maple trees yet they continually travel about, their eye-spots 

 in front according to the general custom of animals. It is easy to tell John Doe 

 from a rose bush, but it is hard to tell whether green flagellates are plants or 

 animals. They fit partly into each kingdom, not wholly into either. 



Sizes and Numbers. All protozoans are minute. Only the larger ones are 

 visible to the naked eye; a colony of Volvox only large enough to be a dot of 

 green; Stentor coendeus to show its trumpet shape; the giant ameba {Chaos 

 carolinensis) of the laboratories to look like a minute splash of water, and the 

 white Spirostomiim ambigumn to cover a hyphen on this page (Fig. 21.2). 

 In general, the largest protozoans are marine radiolarians and foraminiferans, 

 shell-forming relatives of the ameba. There are great numbers of microscopic 

 protozoans; the parasitic ones are especially minute. In a human red blood cell 

 there may be space not only for one parasite, but for many young ones result- 

 ing from its division. Protozoans outnumber all other animals in individuals 

 and perhaps even in species. Euglenas are scarcely visible to the naked eye, 

 although it is common for countless millions of them to create a green layer 

 on an acre of pond water. 



Distribution and Habitats. Protozoans live in moist and watery places. Many 



