C. F. ROUSSELET ON PRESERVING ROTATORIA. / 



It is used by adding first a few drops to the water in which 

 the Rotifers have been placed, then more and more at inter- 

 vals until the animals are sufficiently narcotized. The different 

 species vary very much in the length of time they require for 

 narcotization ; some patients require to be treated very slowly, 

 others very rapidly, to be able to kill and fix them fnlly 

 extended, and for this reason it is best to treat each species 

 separately. The general rule I follow is to add little of the 

 fluid at first, and then, if the animals continue to expand or 

 swim about, more and more at intervals of a few minutes, 

 until their movements begin to slacken. Most free swimming 

 species, I find, can be killed when still swimming about slowly, 

 but with some it is necessary to wait until the cilia have just 

 ceased beating. A few examples mentioned below will give 

 some more details. In order to ascertain the right moment for 

 killing an animal I have not before prepared, I usually separate 

 one or two individuals, and if these can be killed fully extended 

 with a drop of osmic acid, then the others are also ready. Of 

 course, it is very important to kill and fix the animals before 

 they are quite dead, as swelling and other post-mortem changes 

 begin at once after death. 



My process, then, now consists of narcotizing the Rotifers 

 with above cocain-spirit mixture, killing and fixing with 

 £ per cent, osmic acid for half-a-minute or less, washing out 

 immediately and thoroughly in water for a few minutes to 

 half-an-hour, and finally preserving and mounting in 2| per 

 cent, formalin, or, in some cases, in bichloride of mercury and 

 salt solution. 



Rotifers with shells having high ridges or mouldings, such as 

 Uuchlanis triquetra, Mastigocerca bicristata, Metopedia tripicra 

 and oxysternon, some species of Brachionus, etc., must not be 

 left long in water, as the lorica often swells a little and the 

 ridges and mouldings become more or less obliterated ; such 

 species must be washed quickly in water and transferred at 

 once to the formalin, which preserves the shell perfectly. 



It is very necessary, in order to avoid greater trouble after- 

 wards and make satisfactory mounts, to transfer the living 

 Rotifers first of all into perfectly clean water, free from any 

 particle of foreign matter, living or dead. I keep some clean 

 filtered pond water, and pick out and transfer the Rotifers into 

 clean cells as many times as may be necessary. Small particles 



