of the Fishery Board for Scotland. 



143 



lowest and highest at ten fathoms the difference is 15*0 degrees. In 

 the case of Loch Lomond the difference between the maximum and 

 minimum temperatures at the surface and at ten fathoms at the upper 

 end is respectively 17*4 degrees and 12*7 degrees. The differences in 

 the temperatures of the two lochs will no doubt vary more or less from 

 year to year, but will probably still be considerable. 



Pelagic Entomostraca captured by the Tow-nets. 



The tow-net examination of Loch Arklet yielded some interesting results 

 as regards the seasonal distribution of some of the species. Holopedium 

 gibberum, one of the most remarkable species of the Cladocera in Britain, 

 was moderately common in the tow-net gatherings collected in September 

 and November 1897. When the loch was examined in June 1898 the 

 species was abundant all through the water; a haul was made at different 

 parts, and though the net was only towed for about ten minutes each 

 time, it captured at each haul a large quantity of this Cladoceran. On 

 the other hand, when the loch was visited on the 15th of March not 

 a trace of Holopedium could be seen, though several hauls were made to 

 find out if the species had drifted to any particular part of the loch, or 

 had temporarily left the surface and was living near the bottom, but it 

 had completely disappeared. It may also be mentioned that in June, 

 when the Holopedium was so abundant, other species previously observed 

 were now either very scarce or absent in the gatherings collected, as 

 if they had been more or less crowded out by this particular Cladoceran. 

 Another point of interest may be noticed here as to certain differences in 

 the catches made by the coarse and fine tow-nets. When the loch was 

 examined in March the gatherings collected by both nets were compara- 

 tively small, and Diaptomus gracilis, one of the Copepods captured, though 

 scarce in that collected by the coarser net, was moderately common in the 

 other. When the circumstances connected with these captures are 

 considered, it seems as if the reverse of this might have been expected. 

 The coarser net is fine enough to catch even smaller organisms than this 

 Diaptomus, and as it allows the water to pass through it more freely than 

 the fine net does, a greater quantity of water passes into the net, and it also 

 passes in more quickly, and thus allows free-swimming organisms less 

 time to clear out of the way. With regard to the fine net, the meshes are 

 so fine that the water does not pass very readily through them. This net 

 has therefore a tendency to push the water out in front of it while it is 

 being towed. An Entomostracan has thus a better opportunity to escape 

 being captured by this tow-net than by the other ; yet, as stated above, a 

 greater number of Diaptomus gracilis were obtained by the fine net 

 than by the coarser one. The annexed Table contains the names of all 

 the species captured in the tow nets, and the dates when captured. 



[Table. 



