CEXTRIFUGE IX POND-LIFE WORK. 247 



-except a very small drop at the bottom, and then to suck up 

 this remaining drop and forcibly expel it several times. It is 

 very necessary to carry out this alternate sucking up and expul- 

 sion a few times, as practically all the organisms that have been 

 thrown down are, immediately after the centrifuging at any 

 rate, forming an actual deposit on the glass, and therefore 

 require to be detached by the application of a little spray of 

 water. In the case of the small tubes these operations are 

 most easily performed with a Rousselet " thistle-head " pipette 

 drawn out to a very fine point. 



Obviously the next thing to be done is to transfer a portion 

 of the drop of water, or the whole of it if very small, to a glass 

 iilip, live- box, or compressor. As it is desirable, in order to get 

 the best results with high-power objectives, that the film of 

 water to be examined under the microscope should be as thin 

 AS it is possible to make it, only a very small quantity of water 

 is required. At this stage a further concentration of the 

 organisms can easily be carried out if necessary by taking a 

 drop of water two or three times larger than that actually 

 lequired and allowing it to evaporate to the desired size. In 

 any case it is advisable to allow the drop to stand a few 

 minutes before spreading it out with the cover-glass, as some at 

 least of the organisms settle to the bottom and so do not escape so 

 easily to the margins when pressure is applied to the cover-glass. 



Having now disposed of the purely technical details connected 

 with the use of the centrifuge in the examination of pond and 

 other w^aters, it may be well to consider briefly some of the 

 results so far obtained. As regards the little organisms them- 

 selves it is found that they are mainly representativ^es of the 

 bacteria, schizophyceae, desmids, diatoms and chlorophyceae 

 among the plant types, and heliozoa and flagellates among the 

 animals. With the exception of the bacteria all the forms 

 constituting the genuine " centrifuge plankton " are among the 

 smallest examples of their respective groups, and it is somewhat 

 of a revelation to those who hav^e only collected pond-life with 

 nets to observe what a number of excessively small species, even 

 of such well-known groups as the desmids and diatoms, are 

 to be found living in suspension in the open waters of our lakes, 

 ponds and ditches. In harmony with this mode of life many 

 of the species, especially when non-motile, are provided with 



