292 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



avoid this objection in the second method of volume 

 estimation. 



According to this method the catch is filtered through 

 the finest silk cloth. Filter paper is of no use, as so many 

 organisms remain immovably attached to it. The damp 

 mass of the plankton is then introduced into a glass 

 measuring cylinder, in which an exactly known volume 

 of alcohol is contained. The height of the mixture of 

 plankton and alcohol is now measured. The difference 

 between this and the previous measurement gives us, of 

 course, the volume of the catch. This method is good, 

 but not always practicable because on account of the 

 extremely small catches occasionally made there is hardly 

 any appreciable difference in the two measurements. 

 This method gives what Schiitt terms the " Dichtes 

 Volumen." There is yet another way of arriving at the 

 " Dichtes Yolumen." The volume of the catch and fluid 

 is first of all measured. Then the catch is filtered, and 

 the filtrate measured. The difference gives, of course, 

 the volume. This method is applicable for catches that 

 consist of microscopic material. Hensen considers it gives 

 inaccurate results. The only way of arriving at the true 

 volume of an organism, that is the sum of the volumes 

 of the individuals without the adhering particles 

 of fluid, would be to estimate the average volume of an 

 individual of the species, then count the number in the 

 catch, and multiply the average volume by the number 

 in the catch. This estimation is, however, too difficult, 

 and requires too much time to be practicable. The 

 absolute volume, that is the volume of the dry substance, 

 would give us the most complete idea of the volume of a 

 plankton collection. No method has as yet been devised 

 for its estimation. The second method, that of weight 

 estimation, is very simple, but has gradually been replaced 



