LINNEAN SOCIETY OF LOXDOJ^. 



21 



closing vertical nets, the Nansen and the Petersen-Hensen, a 

 weighted and two surface, open, horizontal tow-nets, all made of 

 No. 20 bolting-silk ; and in addition a coarser silk tow-net (No. 6 

 silk) and a large-meshed shear-net only used occasionally. 



During the present Easter vacation (April 11th to 29th, 

 inclusive) I have taken 186 additional gatherings, in 15 working 

 days (an average of over 12 per day), which will serve to compare 

 with those taken during the corresponding period of 1907. The 

 number of Diatoms does not appear to be so great this year as in 

 1907. The spring maximum does not reach to such a height and 



I I 



OFF PORT ERIN l.o.M. 



£3*3. 



is certainly later in April than was the case last year. I have as 

 yet only the volumes of the catches before me, the numbers of the 

 different organisms present in each net have not yet been calculated. 

 The monthly average in cubic centimetres for the first four months 

 of the year 1908 is as follows : — January 0-8 ; February 0-6 ; 

 March 1-8 ; April 7"4, showing an increase in March which 

 became still more marked in April, but is small compared with 

 that in 1907. The average haul during April, 1908, with the 

 different nets used is : — 



Hensen. Nansen. 



0-6 



1-42 



Shear. 



15 



Showing much the same proportions between the nets as in the 

 previous year, but smaller numbers throughout. 



I now turn to the conclusions to be drawn from a study of the 

 detailed figures for 1907. It is clear that many of the great 

 seasonal variations in the plankton are not due to changes in the 

 sea-water such as are recognised in hydrographic observations, but 

 are caused simply by the normal sequence of stages in the life- 

 histories of organisms throughout the year. No amount of 



