METHODS OF PLANKTON RESEARCH. 543 



Physalia, Pyrosoma, Salpa, Schizopods, Janthina, Beroe, 

 Pteropods. Thus, one of the nets with an opening of 1*13 

 square metres hauled up from a depth of 500 metres 520 

 Pyrosoma on one occasion. The question is — What has 

 brought these together? Neither wind nor their own 

 motion, unless governed in some way unknown to us, 

 could do this. At another place 5,860 Doliolum were 

 caught in one haul of the net, as against 1,500 in all the 

 other catches together. 



Darwin, and other observers, had previously recorded 

 the fact that long stretches of the sea were frequently met 

 with, deeply coloured by the abundance of some animal 

 or plant species, as, for example, Trichodesmium 

 erythraeum. It is this association of planktonic organisms 

 in swarms that is now being investigated by Herdman 

 (17), and it will be interesting to see how far it extends. 

 (See also 19.) 



With the exceptions of some swarms, Hensen main- 

 tains that the equal distribution was never disturbed to 

 such an extent, where the conditions remained the same, 

 as to render the application of the quantitative method 

 unsatisfactory. In the Sargasso Sea, for example, where 

 there is no current practically speaking, the catches were 

 astonishingly small, but the volume remained constant 

 over a stretch of some thousand miles. It is possible, 

 however, that the constitution of the catch was altered. 



The results of this expedition tend to show that the 

 ocean waters are very poor in plankton. There is a sharp 

 distinction existing between oceanic and coastal forms ; 

 many of the oceanic species are never or only exception- 

 ally seen near the coast, and one must visit an oceanic 

 island in order to study them. What is the barrier to 

 this distribution? The oceanic species are neither more 

 frail nor more nor less active than many of the coastal 



