METHODS OF PLANKTON RESEARCH. 549 



Ceratium balticum, C. longipes, C. macroceros, C. fusus, 

 Polychaete larvae, Copepod larvae, Oithona similis, 

 Centropages hamatus, Paracalanus and Pseudocalanus. 

 These are all forms which are characteristic of the North 

 Sea and West Baltic water, where the salt contents are 

 high. Owing to the peculiar conditions prevailing in the 

 Baltic, a great variation occurs in the salt contents of the 

 water, varying from 20 %o in the West to fresh water in 

 the North-east, and, moreover, at any station there is 

 commonly a great difference between the salt contents at 

 the surface and at the bottom. It is, therefore, natural 

 to presume here that the greater abundance of the 

 plankton in the deeper layers was due to the salt contents 

 of the water, since that was greater in these layers than 

 at the surface, and the organisms present were those 

 characteristic of salt water. At the present time, how- 

 ever, a great deal still requires to be learnt with regard 

 to the relations between the plankton and the hydro- 

 graphical conditions, and in many cases the results 

 obtained so far contradict each other. 



Finally, it is necessar}^ to examine some of the 

 extremely interesting statistics given by the Hensen 

 method quoted by Jenkins and others. I refer first to 

 such estimations as the number of Copepods in the West 

 Baltic or the number of Peridinians annually devoured 

 by a Copepod. We have only to consider how little we 

 know of the conditions under which these plankton forms 

 live, and the admitted inaccuracies of the method, to see 

 that such results must be so hypothetical as to be of very 

 little practical importance. 



To one of the calculations I must refer in greater 

 detail. The number of floating eggs of the cod and flat 

 fishes found in the Eckenforde waters, the area of which 

 is 16 miles, was estimated at 30 per square metre of the 



