154 DISCOVERY REPORTS 



net, and to a greater extent the 750-500 m. net, yielded larvae, yet for all stages and 

 whatever the time of day by far the greater majority was to be found in the four upper- 

 most nets between 500 m. and the surface. 



The first three diagrams indicate very clearly the pronounced diurnal migration that 

 is to be found in the Calyptopis stages. It is to be noted that in the period which in- 

 cludes midday there are still larvae to be found in the uppermost net; in Calyptopis 2, 

 for instance, 20 per cent of the total were found in this net. Even with such proportions, 

 however, the three diagrams show that in the daylight hours the greater bulk of the 

 larvae is away from the surface and below 100 m., while at night they are massed in the 

 surface water above 100 m., with a marked concentration between 50 m. and the sur- 

 face noticeable in Calyptopis 2 and 3. In the daytime the mass of the larvae are between 

 250 and 100 m., but the appreciable percentage in the 500-250 m. net indicates that the 

 larvae are concentrated in the lower rather than the upper portion of the 150 m. 

 traversed by the 250-100 m. net. If this is so it means that these larvae must migrate 

 upwards through a distance of roughly 200-250 m. to get to the surface each night. 



Mention has already been made (p. 121 supra) of the distribution of the larvae — 

 Calyptopis 3 predominating — in the South Georgia plankton survey in January- 

 February 1930. It will be recalled that all the larvae were taken in the stations made 

 beyond the 250 m. contour and that the 500-250 m. net was the deep limit for the 

 majority. 



It seems likely that light intensity, rather than temperature or salinity, is the con- 

 trolling factor in the vertical distribution of the larvae, for in their daily migrations the 

 variation experienced by the larvae in the two latter factors is that to be found in the 

 Antarctic surface water and the warm deep water. The absence of larvae in the shallow 

 water in the neighbourhood of South Georgia seems to indicate that there is a necessity 

 for the larvae to seek out greater depths, so that in the daytime they may recede to 

 them in order to obtain that degree of light intensity which is most favourable for their 

 existence. 



In the two diagrams representing Furcilia i and Furcilia 2-4, the paucity of data 

 for the period 0200-0600 upsets the evenness of the curve showing diurnal migra- 

 tion. Of Furcilia i for this period two larvae only were taken, and both were in the 

 surface net: there was one larva of Furcilia 2-4 in the 250-100 m. net. It is probable 

 that both groups of larvae, if they were adequately represented during this four-hour 

 period, would have their greatest concentration between 250 and 100 m. On the whole 

 the two diagrams show, although perhaps less clearly, the same diurnal migration as 

 is witnessed in the Calyptopis stages. 



There is a noticeable difference in the last of the six diagrams, that representing 

 Furcilia 5 and 6, compared with the five preceding it. The larvae at all times of the day, 

 with the exception of the 1400-1800 period, are concentrated in the two uppermost net 

 hauls. The diurnal migration of earlier stages is not evident unless the distribution of 

 the 1 400- 1 800 period is indicative, but whether or not this is so, the diagram shows that 

 in twenty out of the twenty-four hours the larvae are at the surface and that below 100 m. 



