NO. 1803. DIRECTIONS FOR COLLECTING ROTIFERS— BEAVCHAMP. 183 



Their use is, even though apparent!}" complicated, in reahty quite 

 simple. It is necessary to distinguish, ])oth in collecting and subse- 

 quent fixation, two classes of rotifers — the purely pelagic forms, 

 which swim without ever anchoring themselves and ])elong to the 

 true plankton, and the forms which are not good swimmers, but move 

 about among detritus and aquatic vegetation in search of sustenance, 

 not traveling far, and frequently fixing themselves by the toes. We 

 will treat them separately. 



The plankton of smaller bodies of water, ponds, pools, ditches, etc. 

 C'lTeleoplankton" of Zacharias), is periodic, appearing and disap- 

 pearing in a few days or a couple of weeks. The rotifers in such 

 places generally have two periods of especial abundance, one in the 

 spring, the other in the autumn. April-]\Iay and September-October 

 are for the neighborhood of Washington the best times to collect these 

 animals. This is also the time for the appearance of the males. For 

 their ca})ture a small net of what is known as "china silk" is very 

 suitable. The lower end of the net should be tied around the neck of a 

 \\dde-mouthed bottle. In this way the surplus water is filtered off; 

 and the animals remain m the bottle in a small cpiantity of water, 

 which is poured into a suitable bottle for transportation. If procur- 

 able, an aluminum tube, closed at the bottom, is preferable to the 

 wiile-mouthed bottle for use with the net, as it does not break on 

 acciilental contact with stones or other hard ol)jects. No stagnant 

 pool should be neglected, no matter how small or apparently impure. 

 A number of species accommodate themselves to these conditions. 

 Water with abundant organic matter, as, for instance, farm-yard 

 ponds, is the favorite resort of certain species like Hydatina senta, in 

 fact, wherever microscopic algse are abundant. The plankton of" 

 swamps, lakes, and rivers may be collected b}^ the usual methods, 

 although the apparatus can be much simpler. Tliis applies also to 

 marine rotifers occasionally found in great abundance in littoral 

 plankton. 



The fixation of the collected material ma}" without detriment be 

 deferred a few hours, according to convenience. By the aid of a 

 strong magnifier it is ascertained whether the rotifers are present in 

 sufficient numbers to warrarrt the treatment. It is well to concen- 

 trate the animals in the smallest possible quantity of water, as the 

 reagents are rather expensive. This is accomplished quite easily by 

 exposing the collection in a glass jar to a one-sided illumination for 

 half an hour. The rotifers soon assemble in a small, whitish cloud, 

 easily visible to the naked eye, on the illuminated side, near the sur- 

 face, and with a pipette they are transferred to a tube of 2 to 10 

 cubic centimeters capacity for fixation.* 



The narcotizing operation consists in adtling to this tube solution 

 No. 1 in small portions, mixing well each time. The animals at first 



»ir apothecaries' measure is Uocd, it may be noted Unit 1 cc. e(iuals J fluid dram. 



