208 C. ROUSSEf.RT ON A METHOD OP PRESERVING ROTATORIA. 



Bearing in mind all that lias been said with regard to nar- 

 cotizing, killing, fixing, and preserving, I will now give a 

 detailed example of my procedure by explaining how I proceed 

 in preserving Asplanchna priodonta, and then indicate such 

 modifications as are necessary for other Rotifers, for each species 

 has its peculiarities and affinities, and must be killed in a 

 slightly different manner ; for the same reason it will be neces- 

 sary to separate the various forms, as one will hardly succeed 

 in preserving satisfactorily a number of different species at 

 the same time, but few as well as a large number of indi- 

 viduals of the same species can be treated collectively by my 

 method. 



Asplanchna . priodonta is usually found in great numbers. 

 After isolating some scores or hundreds in a small trough of 

 clean water I add coca'in solution by degrees, in all about 

 one-tenth to one-eighth the quantity of the water. This is 

 measured roughly in this way : if I know the trough contains 

 five pipettes full of water, I add one-half pipette full of the 

 solution. The Asplanchna wHU not mind the cocain at first, 

 but after a time their movements will become slower, and in 

 about half-an-hour's time they will have sunk to the bottom of 

 the trough. The trough is then transferred to the stage of the 

 microscope, where it will be seen that the animals are perfectly 

 extended, weakly vibrating their cilia, but otherwise motionless. 

 In order to find out the right moment to kill them, I take out 

 a few from time to time on a slide and add one drop of the 

 fixing solution. As soon as they allow themselves to be killed 

 without contracting a small pipette full of the fixing solution 

 is run down the sides of the trough ; the solution being heavier 

 than the water will spread in a layer on the bottom of the 

 trough, covering the Rotifers, killing and fixing them, the 

 majority in a perfectly extended state. They are left there for 

 fifteen minutes, and then must be washed thoroughly with five 

 or six changes of distilled water, either in the same trough or 

 in a test tube, to remove all trace of the fixing solution. The 

 animals will then be ready for putting up in preserving fluid. 



Asplanchna priodonta contracts slowly just before dying in 

 the cocain solution, and must, therefore, be killed quickly, and 

 before this contraction takes place. Asplanchna Brightwellii 

 dies in a perfectly extended state, but must also be fixed before 

 being quite dead to prevent post-mortem changes. 



