THE WHEEL ANIMALCULES (ROTATORIA) 583 



all the organs are sharply defined, so that they are readily seen. 

 After death, the transparency and sharpness are as a rule lost, and 

 naost methods of kilhng the rotifers cause them to become strongly 

 contracted, so that the structure is no longer clear. Direct observa- 

 tion of the living animal will therefore always remain one of the most 

 important methods of studying these forms, for whatever purpose. 

 By Rousselet's method, however, it is now as easy to pre- 

 serve most of the Rotifera in natural form as any other lower 

 animals. This method is essentially as follows: The animals 

 are killed uncontracted by the aid of a narcotizing fluid, the essen- 

 tial feature of which is a ^ to i per cent solution of hydrochlorate 

 of cocaine. The cocaine may be used in a simple watery solution, 

 but better results are reached by using the following mixture: 



Hydrochlorate of cocaine (2 per cent solution) . . 3 parts 



Methyl alcohol i part 



Water 6 parts 



The rotifers are brought into a small volume of water, and a 

 little of this narcotizing fluid is mixed with it. The proper amount 

 must be learned by trial, but it is always best to begin with a very 

 small proportion of the fluid, ^V or less, and to add more as required. 

 This fluid causes the rotifers to swim slowly and gradually to sink 

 to the bottom. They will soon die, and if allowed to die unfixed 

 will be quite worthless for study, destructive changes taking place 

 in the tissues at the moment of death. As soon therefore as the 

 ciliary movement has nearly ceased, as much as possible of the 

 water should be drawn off, and a small amount of 0.25 per cent 

 osmic acid introduced, which kills and fixes the rotifers at once. 

 Now the osmic acid should be drawn off at once and water added 

 or the rotifers removed with a capillary pipette to fresh water; 

 they should be washed several times in distilled water. If the 

 osmic acid is allowed to act too long the rotifers will be blackened. 

 The blackening may, however, be later removed, if necessary, with 

 hydrogen peroxide. After washing, the rotifers should be pre- 

 served in 3 to 6 per cent formaUn. They cannot as a rule be pre- 

 served in alcohol without extensive shrinkage, rendering them use- 

 less for further study. 



