30 



THE ART OF MAKING MICROSCOPE SLIDES 



Rotifers 



tance because satisfactory wholemounts 

 of rotifers cannot be made in either resin- 

 ous or gelatinous media, since no method 

 of dehydration has yet been discovered 

 which will not distort all save a very few 

 of the toughest rotifers. 



The collection of rotifers is relatively 

 simple. Planktonic forms, either marine 

 or fresh-water, may be taken in fine 

 plankton nets and usually occur in con- 

 siderable quantities where they occur at 

 all. The tube or bottle at the end of the 

 plankton net should be emptied into a con- 

 siderable volume of water and kept well- 

 oxygenated unless the specimens are to 

 be prepared immediately. The usual 

 methods of plankton concentration are 

 very unsatisfactory for delicate rotifers, 

 and it is better to rely on their attraction 

 by light, and by high concentrations of 

 oxygen. If, on the return to the laboratory, 

 the quart or gallon of plankton suspension 

 be placed on a bench and one side shaded 

 while the other is brilliantly illuminated, 

 all the planktonic rotifers will be found to 

 concentrate at the surface on the illumi- 

 nated side of the bottle. They may then be 

 picked out without difficulty with a fine 

 pipet and transferred to a watch glass for 

 narcotization. If there are only a few 

 rotifers present it may be necessary to 

 take the jar into a darkened room and to 

 illuminate one angle of it with a small 

 spotlight (such as the Nicholas lamp used 

 by embryologists) which will collect all 

 the rotifers from half a gallon of water in a 

 few minutes. If the jar is going to be left 

 for some time under these conditions it is 

 desirable to use some form of heat filter 

 between the lamp and the jar. 



The collection of sessile rotifers is more 

 difficult. They will usually be found at- 

 tached to the stems of water plants, and 

 to the underside of water-lily leaves. It 

 has been the author's experience that 

 more rotifers will be found in relatively 

 small ponds than hi large lakes, and that 

 if one could find a body of water several 

 feet deep but of only a few hundred square 

 feet of surface area, and if this water is 

 relatively choked with large water weeds 

 but contains only a small quantity of 

 green algae, it is likely to contain many 

 of the rarer forms of sessile rotifers. The 



distribution of these forms is, however, 

 very scattered and it is scarcely ever 

 worth while to collect large quantities of 

 water weeds with a drag and then to take 

 them back to the laboratory and hunt 

 through them. It is far more profitable 

 to settle down and hunt the weeds as 

 they are in the water, cutting from them 

 short lengths of stem or small areas of 

 leaf which bear the required forms. These 

 are then placed in a large jar of water from 

 the pond and brought back to the labora- 

 tory for further treatment. 



The most difficult part of the prepa- 

 ration of a mount of the rotifer is to 

 narcotize it correctly. Hanley (Micro- 

 scope, 7:155) has discovered that the use 

 of alcohol in Rousselet's fixative is an- 

 tagonistic to the cocaine in the same solu- 

 tion and that it is, therefore, by Rous- 

 selet's method necessary to use very large 

 quantities of narcotic with a resultant 

 very short interval between complete 

 narcotization and death. With Hanley's 

 narcotic the narcotization is relatively 

 rapid but the interval between complete 

 narcotization and death is relatively long. 

 With Rousselet's fixative there is often 

 only a period of from one to two seconds 

 between the moment when the fixative 

 can be applied and the moment when the 

 rotifer dies and is then worthless. With 

 Hanley's narcotic, this period is extended 

 for as long as 10 to 15 seconds and only 

 those who have mounted rotifers by 

 Rousselet's method can appreciate how 

 great is this advantage. 



For the actual process of narcotization 

 it is necessary to have two watch glasses, 

 one containing the rotifers swimming in 

 their normal environment, and the other 

 a 10% solution of formaldehyde. These 

 two watch glasses should be sufficiently 

 far apart that fumes from the formalde- 

 hyde do not dissolve in the glass contain- 

 ing the rotifers. There is also required a 

 supply of Hanley's narcotic, a fine pipet, 

 and a dissecting microscope having a 

 power sufficiently high to enable the 

 rotifers to be seen clearly. For an average 

 watch glass containing the rotifers two 

 drops of Hanley's narcotic are added to 

 the water and mixed by sucking the water 

 in and out with a rather coarse pipet. 



