342 DISCOVERY REPORTS 



found that diatoms, one month after mid-summer, made from five to seven divisions in 

 three days, and Harvey's (1934) preliminary observations suggest that diatoms in the 

 Plymouth area in April and May divide twice daily. 



In the next section we shall see that changes in the extent of the vertical migration 

 undertaken by animals will result under certain usual conditions of water movement in 

 changes in their horizontal distributional density. We know that vertical migration does 

 vary in extent from time to time, and if these changes were proportional to changes in 

 the phytoplankton, then we should get just such a distributional arrangement of the 

 animals as we have seen in relation to the phytoplankton. 



The problem is one which must be attacked by the experimental method both in the 

 field and the laboratory. 1 My colleague Mr C. E. Lucas has undertaken experiments to 

 investigate the effects of different strengths of phytoplankton concentration upon plank- 

 ton animals, as well as the rates of consumption of the phytoplankton by the animals. 

 Whilst it is too early yet to give a full account of the experiments, which are still in 

 progress, he has kindly allowed me to refer to two interesting series of results. In one 

 set of experiments he had two long tubes, one filled with sea water without phyto- 

 plankton, and the other with sea water of the same character but containing a culture of 

 Nitzschia, the strength of which was varied in different experiments. An equal number 

 of Mysids was placed in each tube, and each tube was darkened for half its length by 

 being covered with light-proof paper. Then at intervals he observed and recorded the 

 numbers of Mysids showing themselves in the uncovered part of the tubes during the 

 daytime. The following pairs of figures represent the numbers of Mysids seen in each 

 tube during each experiment, those on the left refer to the diatom-free tube, and those 

 on the right to the diatom tube. The number of observations in each experiment varied, 

 but their average was eight. 



The experiments shown in the last column were made with young Mysids, the remainder 

 with adults. 



In another series of experiments he kept an equal number of Copepods or Mysids in 

 a number of different jars containing sea water with increasing strengths of Nitzschia. 

 He found in seven out of eight such experiments that the animals lived longest in ranges 

 of intermediate phytoplankton. He will later publish a detailed account of the work 

 when the experiments have been carried further. 



1 Professor E. W. MacBride, F.R.S., has recently informed me, and kindly allowed me to mention, 

 that in rearing Echinoderm larvae he has found that if the diatoms {Nitzschia) used as food multiply too 

 rapidly to form a dense culture then the larvae die. 



