548 TRANSACTIONS LIVERTOOL BIOLOGICAL SOCIETY. 



difficult. The total volumes and the constitution of the 

 catches made varies considerably at the different stations. 

 Thus, in one of the expeditions at Station 9, North Sea, 

 there was a greater quantity of Ceratium macroceros and an 

 abnormal number of Oithona and Pseudocalanus. At 

 Station 11 a great number of Actinotrocha larvae formed 

 an important constituent in the catch. In May, 1903, 

 there was in the North Sea a remarkable preponderance 

 of plankton in the upper five-metre layer, far 

 exceeding that of the deeper layers. This was quite 

 independent of the salt contents, for it occurred where 

 there was no difference in the constitution of the sea 

 water between the surface and the bottom. Thus, at 

 one Station the numbers for plant cells were in the pro- 

 portion to 5 metres deep, 400 ; 5 to 40 metres deep, 55 ; 

 40 to 75 metres deep, 8 ; 75 to 150 metres deep, 2 ; 150 to 

 450 metres deep, 1. In another example, however, there 

 was a decrease as above from the surface down until the 

 25-metre depth was reached, but between 25 metres and 

 75 metres deep the average number of organisms present 

 was twice as great as at the surface, that is, about twenty 

 times what it should have been. This was due to an 

 abundance of Phaeocystis. What determines these varia- 

 tions ? Salt contents seem to have nothing to do with the 

 diminution which occurs as one passes from the surface 

 into deep water, though in the Baltic, as will be mentioned 

 below, the salt contents seem to cause an opposite result. 

 Light intensity might be connected with it, but very good 

 catches are often obtained at depths of 75 to 100 metres. 

 In the Baltic, in February, 1903, the figures gave different 

 results for the vertical distribution, for the plankton was 

 always more abundant in the deeper layers than at the 

 surface. 



The organisms which caused this increase were 



