206 C. ROUSSELET ON A METHOD OF PRESERVING ROTATORIA. 



to contend with is the killing in an extended state ; few other 

 animals can contract into such a shapeless mass when we 

 attempt to kill them by ordinary means, such as alcohol, 

 poisons, heat, etc. ; even the quick acting osmic acid is not quick 

 enough to prevent a complete collapse. It is, therefore, neces- 

 sary to have recourse to a narcotizing agent which will act very 

 slowly and paralyze the nerves and muscles sufficiently, that 

 when the killing fluid is added the animals will no longer be 

 able to contract. Such a narcotizing fluid, eminently suitable 

 for Rotifers, has been found in a weak watery solution of 

 hydrochl orate of cocain of 1 to 2 per cent., first proposed by E. 

 F. Weber for keeping quiet very active Rotifers when under 

 observation, and for which purpose it answers admirably. If 

 a small quantity of this solution be added to the pond water in 

 which the Rotifers are, they will at first not be affected at all, 

 but continue to swim about as usual. After some minutes 

 (5 to 15) their motion will become slower and slower, and in 

 the most successful cases they will finally sink to the bottom of 

 the trough fully extended, with the cilia vibrating but feebly. 

 The Rotifers will not be dead yet, and if an attempt be made 

 to kill them at once in that state they will most likely contract 

 and be spoilt. It is necessary to watch them under the micro- 

 scope until the cilia have just ceased to vibrate, and then, at 

 least in the majority of species, is the right moment to kill 

 them, as explained below. The action of cocain varies greatly 

 in different Rotifers ; some species, such as Asplanchna, can 

 stand a good deal of the anassthetic, while others, such as 

 Stepbanoceros, are extraordinarily sensitive to it. The 

 quantity of cocain added to the water must, therefore, be 

 varied with every species according to requirements taught by 

 experience. As a general rule I can say, add as little as 

 possible, but sufficient to kill in about an hour's time ; if the 

 animals collapse, or show signs of weakness at once, it proves 

 that too much has been added. 



Killing and fixing. — When the Rotifers have been sufficiently 

 long under the influence of the cocain they are killed with 

 Flemming's chrorao-aceto-osmic acid mixture,* which fixes them 



* Flemmingr''* fixing solution consists of — 



1 per cent, chromic acid 15 parts 



2 „ osmic acid 4 „ 

 Glacial acetic acid 1 part 



