584 FRESH-WATER BIOLOGY 



If desired, the specimens may be permanently mounted in hollow 

 ground slides. The slides should be thin and the concavities shal- 

 low, so that high powers of the microscope may be used. The 

 specimens are transferred to the concavities along with some of the 

 formalin and covered with a circular cover-glass. It is best not to 

 leave any bubbles of air beneath the cover. The superfluous for- 

 malin may be withdrawn from the edge of the cover with a bit of 

 filter paper, and the cover is then sealed by the aid of a revolving 

 stage. It is, of course, necessary to use some sealing material that 

 will not allow water to evaporate through it. Rousselet recommends 

 the following for sealing the mounts: After fixing the cover with a 

 layer composed of a mixture of two-thirds gum damar with one- 

 third gold size, there are added two coats of pure shellac, followed 

 by three or four coats of gold size, allowing twenty-four hours for 

 each coat to dry before another is added. 



Extensive collections containing many species of Rotifera may 

 be made by travelers and others by the use of the method given 

 above. The rotifers, taken with a net or otherwise, are brought 

 into as small an amount of water as possible, in a watch-glass. 

 Then a considerable quantity of the narcotizing fluid is introduced 

 and the rotifers are watched till most of them have sunk to the 

 bottom. Thereupon the water is removed, so far as possible, and 

 the j per cent osmic acid added. This is removed as quickly as 

 can be done without taking up too many of the rotifers; they are 

 then washed and preserved in formalin. It is very desirable to kill 

 a certain proportion of every collection in osmic acid without previ- 

 ous narcotization, as some of the loricate rotifers are more easily 

 determined from contracted specimens than from extended ones. 



A method of mounting Rotifera in the ordinary mounting media, 

 such as Canada balsam, has been given by Zograf . It does not give 

 such perfect results, in most cases, as does Rousselet's method, but 

 it is useful for some species. The rotifers are stupefied and killed 

 in the way given above. After they have been in the osmic acid 

 from two to four minutes, this is removed as far as possible, and a 

 considerable quantity of 10 per cent pyroligneous acid is added. 

 This is allowed to act five or ten minutes; then the rotifers are 

 washed several times in distilled water. As a result of the harden- 



