Long and slender Spirostotnum, a 

 giant among protozoans, dwarfs the 

 smaller, slipper-sliaped Paramecium. 

 Two rotifers in this same microscope 

 field are many-celled animals, yet are 

 barely larger than the paramecia. 

 ( General Biological Supply House, 

 Chicago ) 



the space for a jar of water from a bird bath, a stag- 

 nant pool, or the plant-invaded edge of a pond. 

 There are known to us roughly thirty thousand spe- 

 cies of protozoans, and new ones are reported almost 

 every day. But there are also presumably respect- 

 able ones that are suddenly dispossessed of their 

 status and have to move in with their relatives be- 

 cause they are shown not to be different enough to 

 be considered separate species. 



Size 



Protozoan predominance in number loses some of 

 its overwhelming impressiveness when we consider 

 that almost all protozoans are minute and that most 

 of them are microscopic. The smallest forms, para- 

 sites that live within other animal cells, are only 2 

 microns (1 micron = i/{.r,.(Mio of an inch) in their 

 longest diameter. To learn much about the structure 

 of such animals is difficult even for the most experi- 

 enced microscopists using the best microscopes. For- 

 tunately, most of the free-living forms come in larger 

 packages, but even these are invisible to the naked 

 eye except when a colored species multiplies so fast 

 that through sheer density it colors sea water pink, 

 a rain pool blood-red, forms green scum on ponds, 

 or gives a pink or greenish cast to large snow banks. 



Paramecium caudatum is of moderate size (180 to 

 300 microns or Vssn to 'idn of an inch) and can 

 barely be seen by the unaided eye as a white speck 

 darting about in a dish of pond water. Ten times 

 larger than this are such fresh-water giants as Spi- 

 rosiomum and Stentor, which often measure more 

 than •■'{.-, of an inch. Even these are dwarfed by the 

 shelled foraminiferans of marine waters. If we ad- 

 mit to the phylum Protozoa the slime molds (the 

 Mycetozoa), which many botanists classify as fungi, 

 then these super-amebas, with hundreds of nuclei 

 but without cellular partitions, are by far the largest 

 protozoans. During the multinuclear stage the ame- 

 boid body may extend for several feet as it crawls 

 slowly over a rotten log on the forest floor. The size 

 of any particular protozoan may vary with nutri- 

 tional state and with changing conditions in the en- 

 vironment. It depends also upon consistent heredi- 

 tary differences that mark the many races or strains 

 of any one species. So size is not always a depend- 

 able criterion for identifying a protozoan. Neverthe- 

 less, a fairly definite adult size does characterize 

 each species, as well as each stage of its life cycle. 



Gross Structure 



Though it will save time to consider the protozo- 

 ans as a whole before going on to separate accounts 

 of the classes, generalizations come hard about a 

 group that matches all of the rest of the animal king- 

 dom in its range of sizes, shapes, habitats, structural 

 specializations, feeding habits, and life cycles. Only 

 the basic body plan brings all of these extraordinarily 

 varied creatures into a single grouping. The body 

 consists of one undivided mass of living substance, 

 or protoplasm, bounded by an external membrane 

 that regulates exchanges of materials with the out- 

 side environment. Near the center of the proto- 

 plasm is a formed body, the nucleus, which is in con- 

 trol of essential chemical processes. If an ameba is 

 deprived of its nucleus by accidental or experimen- 

 tal manipulation, the part without the nucleus may 

 move about for a time, but it cannot feed and it 

 soon dies. There is usually only one nucleus, but 

 when there are two or more, no one nucleus is in 

 sole charge of any particular portion of the proto- 

 plasm. In some species the division of the original 

 mass results in a group of cells that remain attached 

 to each other as a protozoan colony. Such a colony 

 differs from a multicellular body in that the cells are 

 usually all alike except during reproduction and in 

 that any one can live independently of the others. 



Body Symmetry 



Protozoans come in every major type of symmetry 

 known in the animal kingdom. In this they differ 

 from all other groups, for in each multicellular phy- 

 lum the members are consistent in having some one 



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