exactly resembles an angular grain of sand and may be blown as a particle 

 of dust to a point far from its original home. Many of the Protozoa profit 

 by this so'called resting period to divide their living substance into two, four, 

 eight, or even more equal portions, so that, when danger is past and the en' 

 vironment is once more suitable for life, the wall of the cyst breaks and not 

 one but a group of Protozoa is released. Under favorable conditions most 

 Protozoa reproduce by dividing into two approximately equal portions, which 

 soon round out to be complete animals, each capable of growing and repeating 

 this process of fission. Thus the number increases in geometrical progression, 

 the population doubling with each generation. It is sometimes said that the 

 protozoan is potentially immortal, since the parent becomes its two offspring — 

 and so on, indefinitely. Some ciliates at intervals each temporarily unite 

 with another for exchange of nuclear material — a procedure known as con' 

 jugation — before fission is resumed. At times in some Protozoa another 

 method of reproduction occurs. One individual may divide into a number of 

 small units, and another one of the same species into a few large units. Neither 

 a large nor a small unit can develop further by itself, but must fuse with one 

 of the other type. The large ones are called macrogametes and are regarded 

 as female, the small ones or microgametes as male, and so appears the be- 

 ginning of sexual reproduction. Isogametes or gametes of equal size are 

 produced by some species. Many of the invertebrates retain the habit of 

 asexual reproduction, but the sexual method, with its possibilities of new com- 

 binations of parental characters, becomes increasingly more important in the 

 higher forms. 



STUDY OF PROTOZOA 



Collecting 



Protozoa large enough to be seen with an ordinary compound microscope 

 may be found in almost any body of standing water. Many forms also occur 

 in soil, but are less easily observed. Even garden bird'baths often prove to 

 be good collecting spots, as many forms may be carried on the feet of birds. 

 The scum on the surface of still ponds swarms with Protozoa and other minute 

 animal and plant organisms. The ooze on the bottom of such ponds is also 

 well supplied with microscopic fauna and flora. 



As in all kinds of hunting, the protozoologist must be somewhat of an 

 ecologist for successful collecting. Almost all Protozoa profit by the presence 

 of enough vegetation to slow down the surface currents and to keep up the 

 oxygen supply. If one is seeking the free-swimming ciliates or flagellates, he 

 will find many kinds at or near the surface of the water or among strands of 

 filamentous algae. Some attach themselves to leaves of submerged plants. 

 Often what appears superficially to be a coat of fine mold or "fur" on such 

 leaves will be revealed by the microscope to be a host of the bell animalcules, 



30 



