these has died out, amoeboid forms may become numerous for several days. 

 Finally some of the smaller meta^oans, such as rotifers and annelids, displace 

 them. 



If observation of these animals under the microscope is difficult on 

 account of their extreme activity, there are several methods of slowing them 

 down. A small drop of sugar syrup, of glycerine, alcohol, or of one per cent 

 solution of formaldehyde placed against the edge of the cover glass under 

 which the animals are imprisoned will gradually seep in and slow up their 

 activity. If cilia and flagella are hard to see, after all possible adjustments 

 of the microscope mirror and diaphragm, a very little weak iodine solution 

 may be allowed to work in under the cover slip and, if used in moderation, 

 will not kill the animals until sufficient time has elapsed for close observation. 

 Cultures 



Many kinds of Protozoa can be grown readily in cultures. Although 

 it takes great care and skill to grow only one kind of Protozoa and maintain 

 them as a pure culture, the beginner will find it relatively easy to establish and 

 maintain mixed cultures. Many and detailed are the directions given in 

 scientific reports, but the root of the matter is that bacteria are necessary for 

 food for many Protozoa, and the larger Protozoa often feed on their smaller 

 relatives, so that any mixture which will develop a growth of bacteria is 

 likely to support Protozoa. The early microscopists noted the appearance of 

 small animals in infusions of hay, bread, and even red pepper, and called 

 these animals Infusoria, a term now reserved for one group of Protozoa. 



Just as each kind of Protozoa has an optimum temperature and light 

 exposure, so each has an optimum of food concentration and an optimum of 

 acidity or alkalinity. These optima account for the succession one finds in 

 ponds and cultures. Unless these optima are known, it is advisable that 

 several cultures, differing in kind and concentration of food, be tried. The 

 materials most commonly used for cultures are timothy hay, wheat and rice. 

 These are boiled for a few minutes and then put in pond water at about the 

 proportions of three inches of hay stem and three grains of wheat or rice to 

 100 cubic centimeters of water. After three or four days a bacterial scum 

 may be seen on the surface and, if no Protozoa are present, some should be 

 introduced. These proportions are about right for Paramecium. Amoeba 

 requires a much lower concentration, Euglena a higher concentration. Rice 

 appears to be somewhat better than wheat for the chlorophyll-bearing Protozoa. 

 These cultures are usually at their best when from two to four weeks old. 

 New cultures may be made at intervals, or old ones rejuvenated by adding 

 small amounts of bread or dried lettuce. These suggestions are given for 

 the interested amateur. For serious work, regular bacteriological techniques 

 are followed, often to the point of raising cultures of particular kinds of bac' 

 teria for food. For detailed information the reader is referred to the papers 

 and books on culture methods listed in the bibliography. 



32 



