308 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



depth, therefore there would be 151'5 x 9 such individuals, 

 that is, 1,364. 



In order to convey some idea of the laborious nature of 

 this method of enumeration, it may be stated that Hensen 

 himself says that he took a week, working eight hours 

 a day, to count the organisms in a single catch. An 

 important question here suggests itself. Is it likely that 

 such an enormous expenditure of energy can be justified 

 by the results obtained ? A perusal of the results already 

 obtained, a short summary of which is appended, is the 

 best answer to the question. The obstacles in the way of 

 a successful counting are numerous, but Hensen has 

 attempted in a masterly manner to overcome them, and 

 if one considers the success which has attended the solu- 

 tion of equally difficult problems, for instance, the 

 enumeration of the corpuscles in the blood of man, there 

 seems no reason to doubt that the Hensen planktonic 

 method, if carefully employed, is a success. 



All reckonings according to this method are so made 

 that they can safely be regarded as minimum totals, and 

 it is absolutely certain that in every case the fertility of 

 the sea is really greater than indicated. Numerous 

 control experiments have been made, and it is proved that 

 with ordinary care the error never exceeds 20 per cent., 

 and should on the average not exceed 7 or 8 per cent. 

 These control experiments were partly so arranged that 

 at the same spot several observations were made one after 

 the other, the vertical net being in every case lowered to 

 the same depth (comparative trials), or the same net would 

 be successively lowered at the same spot to different 

 depths (depth trials). The first series were useful to 

 determine the errors likely to arise in an individual 

 estimation. The second series served to prove that the 



