THE LARVAL STAGES 113 



life-span of each developmental phase in the plankton, (b) the corresponding gatherings based on 

 positive records only, the stations involved in the construction of both (a) and (b) being shown by 

 the bracketed numbers. 



Both presentations of the data reveal (i) a heavy massing of the total free population in the 

 top 100 m., (2) that such vertical migration as is going on does not by any means seem to involve the 

 total surface population, (3) that it is taking place, or seems to be taking place, principally in the 

 1 00-0 m. layer and does not involve either the night time or day time population in any mass descent 

 into the warm core of the deep current flowing counter to the surface stream. 



Table 21. Decline in total numbers of larvae taken below 250 m. as development proceeds, the figures 

 in brackets showing the numbers of stations worked during the life-span of each grouped developmental 

 phase 



Table 22. Vertical distribution of total free population of grouped Calyptopes and Furcilias expressed as 

 average numbers taken per 250 m. vertical haul, the asterisks indicating average gatherhigs of less than one 



(«) 

 Daylight Darkness 



The vertical movement that seems to be taking place seems to be confined principally to the Calyp- 

 topis and early Furcilia stages, but not to Furcilias 4-6. They (the late Furcilias), it seems, tend to be 

 gathered night and day in the top 50 m., where in view of the relatively small numbers we have 

 recorded, it is distinctly possible they are assembled in shallow 'rafts' (pp. 234-6) through which 

 the vertical nets fish only momentarily. 



Finally, to conclude this survey, the question might well be asked, are the conventional methods 

 employed in portraying diurnal vertical migration when the net hauls involved are of variable duration 

 in every instance justified? In other words are we always on safe ground when resolving vertical 

 hauls of 50 sec, 150 sec. and 250 sec.,^ into hauls of a uniform 250 sec. duration? This practice is 



^ Our vertical nets were hauled at the rate of a metre a second. 



