cialized structures. It extrudes any undigested food, 

 and then shrinks by expelling water. Finally it se- 

 cretes around itself outer and then inner cyst walls. 

 Many encyst also when they are regenerating in- 

 jured parts, reproducing, or simply digesting a big 

 meal. Dry cysts have in extreme cases been shown to 

 be capable of returning to active life again at any 

 time during half a century if proper conditions are 

 supplied. For most species the period of viability 

 lasts only for several months to several years. En- 

 cystment is characteristic of parasitic protozoans, 

 for these must temporarily leave their comfortable 

 berths inside moist and nutritious hosts in spreading 

 the species from one host to another. In the oceans 

 encysting protozoans are rare, for the tremendous 

 volume of the marine habitat acts as a great stabiliz- 

 ing mechanism against changes of any kind. Bodies 

 of fresh water are smaller and so less stable as 

 aquatic environments. In temporary ponds and 

 marshes protozoans regularly encyst and excyst with 

 the round of dry and wet seasons. Not all forms can 

 do this, and among the exceptions, as far as we 

 know, is the familiar Paramecium caudatum. 



The capacity to encyst has opened to protozoans 

 a tremendous assortment of land-based but irregu- 

 larly moist niches which would otherwise be too un- 

 reliable for aquatic organisms. Such are the bark on 

 the shady side of trees, the cavities of insectivorous 

 plants and of cup-shaped flowers, the axils of leaves, 

 the crevices in beds of moss, and the surfaces of 

 grasses and other vertical vegetation that are regu- 

 larly wet by dew. A special fauna inhabits the freshly 

 laid feces of animals, remaining active until the sun 

 bakes the feces dry, then encysting again. 



Nutrition 



The protozoan approach to nutrition runs the en- 

 tire gamut of possibilities. There are green flagel- 

 lated forms able to use the energy of sunshine to 

 synthesize their food, like any green plant, from sim- 

 ple materials in water and soil. And there are color- 

 less protozoans that roam, chase, and capture prey 

 like any carnivorous animal. Between these wholly 

 plantlike or wholly animal-like methods are a series 

 of intermediate solutions to the problem of earning 

 a living. Some forms absorb already synthesized and 

 dissolved foods through their external surface and 

 are known as saprozoic feeders. These include many 

 free-living flagellates as well as most of the parasitic 

 protozoans. Others turn from "independent" or pho- 

 tosynthetic habits to saprozoic feeding when occa- 

 sion permits, thus availing themselves of an alterna- 

 tive source of food whenever it presents itself. By 

 far the greatest number of free -living protozoans 

 earn their living by ingesting whole organisms or 

 large particles of organic debris. They feed on bac- 



teria, yeasts, algae, wood particles, and small ani- 

 mals, either other protozoans or certain small meta- 

 zoans. 



Reproduction 



Reproduction in the protozoans is essentially the 

 same as in the multicellular groups, for in all animals 

 the basic process is cell division. Sexual processes 

 are widespread among protozoans, and in some spe- 

 cies must take place at intervals or the strain will die 

 out, but they do not occur in all species. As far as 

 we have been able to determine. Amoeba proteus, 

 for example, has only asexual reproduction. When 

 the animal has reached a certain size and maturity, 

 it divides into two cells, each containing half of the 

 nucleus and of the hereditary materials of the nu- 

 cleus. This division of the parent cell into two halves 

 (binary fission) is the most common method of re- 

 production in protozoans. Two other main types of 

 asexual division are known. One is budding, in which 

 the parent cell retains its individuality while produc- 

 ing, by division, one or more "daughter" cells, usu- 

 ally much smaller in size and less differentiated than 

 the parent. Either before or after it is freed, the bud 

 grows to resemble the parent in size and structure. 

 Budding is typical of the Suctoria but is rare in other 

 groups. Multiple fission, or sporulation, is an asexual 

 process in which the nucleus divides many times, 

 and then the protoplasm divides into as many off- 

 spring as there are newly formed nuclei. This is the 

 protozoan version of mass production, and it results 

 in extremely rapid multiplication. It is seen espe- 

 cially in forms like the sporozoans. Through the vari- 

 ous asexual processes a species is assured rapid mul- 

 tiplication and the maintenance of its numbers. 

 Through sexual processes there arises a steady sup- 

 ply of new variants, individuals with new combina- 

 tions of hereditary characteristics. Each sexually pro- 

 duced individual has the possibility of being better 

 adapted in some way than were either of its parents. 

 Thus sexual processes provide the hereditary varia- 

 tions upon which natural selection may act. Their 

 significance for adaptation and evolution is the same 

 in the protozoans as in higher animals. In animals as 

 small as protozoans growth and reproduction take 

 place on a time scale measured in hours, not years. 

 Paramecium may undergo binary fission as often as 

 three times a day, the smaller ciliate, Glaucoma, 

 eight times a day. 



Behavior 



Anyone who observes the speed with which pro- 

 tozoans dart backward after striking an obstruction, 

 or the persistency with which they squeeze through 

 a narrow passageway between two algal filaments, 

 or the ingenuity with which a sluggish ameba cap- 

 tures a fast-moving ciliate, will want to credit proto- 



20' 



