8 C. F. ROUSSELET ON PRESERVING ROTATORIA. 



become readily attached to the cilia of Rotifers when dead, and 

 it is then often very difficult to remove them ; for the same 

 reason it is not advisable to mount small species in the same 

 cell with larger ones. 



Instead of micro-troughs, as recommended in my first paper, 

 I now use small square blocks of glass, with a hollow ground in 

 and polished, as much more convenient for all the necessary 

 manipulations. These blocks can be placed under the lens of 

 the dissecting microscope,* or the compound microscope, if 

 necessary, and the animals can be watched more closely, which 

 is indispensable with the smaller species. For the purpose of 

 washing, etc., I transfer the Rotifers from one glass block to 

 another by means of a small and very fine pipette, funnel- 

 shaped at one end, the funnel covered with an india-rubber 

 membrane. I have had such pipettes made of various sizes, 

 and can recommend them as the best yet devised for picking up 

 small animals of all kinds in water. 



In killing it is merely necessary to introduce a drop of osmic 

 acid on to the animals under water, and then almost immedi- 

 ately transfer them to some fresh water in another block kept 

 ready, and then again to two or three more lots of distilled 

 water, so as to get rid of all traces of the acid, and finally in 2 J 

 per cent, formalin. 



The following notes will give an idea of the treatment some 

 Rotifers require, and serve in some measure as a guide to the 

 treatment of other species : — 



Stephanoceros and Floscules. — Although I had been occa- 

 sionally successful in preparing a few of these Rotifers, I have 

 only quite lately found the means of killing them fully extended 

 with some degree of certainty. I will describe the modus 

 operandi with regard to Stephanoceros; the Floscules must be 

 treated similarly, but are more difficult. Before beginning 

 the operation, cut and trim a very small piece of the weed to 

 which Stephanoceros is attached, ready for mounting, and place 

 it in a cell of perfectly clean water; then transfer the animal 

 to a hollow-ground glass slip, the hollow of sufficient size and 

 depth, in three drops of water, to which one drop of the 

 narcotizing fluid has been added. After five minutes the 



* A dissecting microscope of some kind is necessary ; my tank micro- 

 scope, provided with an aplanatic lens of G or 10 power, can readily be 

 adapted as a dissecting microscope by screwing it to a suitable stand with 

 arm rests. 



