METHODS OF PLANKTON RESEARCH. 501 



to investigate quantitatively the planktonic fauna and 

 flora, the source of food for the larger sea animals. 



With these facts in view, Hensen invented the nets 

 and methods by which the German investigators have, 

 since that time, diligently worked. These methods were 

 described by Dr. J. T. Jenkins in the Trans. Liverp. Biol. 

 Soc. for 1901, and the purpose of the present paper is to 

 bring the description of the German Plankton methods 

 up to date and to briefly discuss some results of the work. 

 I have been in a fortunate position in this respect, that I 

 have been able, myself, to handle the apparatus whilst 

 participating in the actual expeditions in the North Sea 

 and Baltic. I must thank Professor Brandt for his 

 kindness in securing permission for me to travel on the 

 German Investigation Steamer " Poseidon," and Pro- 

 fessors Lohmann and Apstein for their ever willing help 

 and explanations. 



Essentially from first to last, Hensen and his co- 

 workers have had one aim in view — the better deter- 

 mination of the " see-saw " of life in the water and 

 the laws governing this. It devolved, therefore, into 

 a determination of what plankton was to be found in the 

 sea at a given time and place, and how this mass changed 

 with the change of time or of place, or of both, in quantity 

 and quality. How far this aim has been realised will be 

 discussed after an account of the nets and apparatus used.* 



The Plankton Nets. 



As described by Jenkins, the quantitative net was 

 the apparatus which Hensen invented as the most satis- 

 factory means by which the organisms in a known 



* The blocks for figs. 1, 3, 4 and 7 have been kindly lent by 

 the Commiss. f. wissensch. Meeresuntersuchung. The apparatus 

 described in this paper for the quantitative work is manufactured by 

 Zwickert, Optician, Kiel. 



