Preserving and Mounting Hotifera 197 



is a 1 per cent, watery solution of hydrochloride of eucaine, 

 recommended by Mr. G. T. Harris, for infusoria and other 

 animals. These narcotics, even so dilute, are not to be 

 used pure, as they would cause the rotifers to contract at 

 once and not expand again. The principle to be followed 

 throughout is to use the narcotic so weak that the animals 

 will not mind it at first, but continue to expand or swim 

 about freely. After a short time its effect will make itself 

 felt on their nervous system, and then some more of the 

 narcotic may be added, until complete narcotization is pro- 

 duced, or until the animals can be killed without con- 

 tr actings. 



But before the operation of narcotizing is begun, it is 

 very necessary to isolate the rotifers in perfectly clean 

 water. The best way is to pick them up under a dissecting 

 microscope by means of a very finely drawn-out pipette, 

 having a funnel-shaped enlargement at the other end, 

 which is covered with an elastic membrane. This pipette 

 forms a most delicate syphon, by means of which any 

 selected rotifer can readily be taken up with the least 

 quantity of water, and transferred to another trough or 

 watch-glass full of clean water. This preliminary precau- 

 tion is necessary, because particles of dirt in the water 

 readily attach themselves to the cilia of dead rotifers, 

 rendering them unsightly under the microscope. Another 

 advisable precaution is to separate the different species, 

 because most species require a slightly different treatment, 

 and because the small species too readily adhere to the 

 cilia of the large species. 



Having then isolated a number of free- swimming rotifers 

 in a watch-glass half full of perfectly clean water, one drop 

 of one of the above narcotics is added and well mixed. 

 After five or ten minutes, if the animals continue to swim 

 about freely, another drop is added, and so on until the 

 effect of the narcotic becomes visible, and until the motion 

 of the cilia, or the movements of the animals slacken or 

 almost cease, when they are ready for killing. The effect 



