NATURAL HISTOET OP LAKE UBMI. 357 



of Artemia are occasionally to be seen at certain places near the 

 shore. In such streaks in which the Artemias were more than 

 ordinarily numerous, there were always a large proportion of dead 

 individuals or of individuals with impaired swimming powers ; 

 the greater density of their aggregation seemed to be the 

 result of local currents. In the clear green waters brightly 

 illumined by the sun, the delicate cuticles of the Artemias sparkle 

 like star-spaogles. In order to obtain data for comparing the 

 density of population of Lake Urmi with that of other lakes, a 

 tow-net of silk bolting-cloth (meshes '25 mm. square) was slowly 

 drawn horizontally through about 35 yards of water in the middle 

 of the island archipelago, and again in a vertical direction through 

 25 feet (the utmost obtainable at the station) ; the organisms 

 captured were put into alcohol, their numbers were counted and 

 their apparent volume measured. The two horizontal fishings 

 gave results not differing from the mean by more than 3 per cent. 

 The cubic metre of water was found to contain L577 individual 

 Artemias — or roughly, an Artemia to every pint of water. The 

 preserved material, after being allowed to settle for 24 hours,^ 

 occupied an apparent volume of 42*5 cubic centimetres. The 

 vertical fishings gave rather lower results. Six hauls of the 

 plankton-net showed that the average vertical distribution was 

 about 1203 Artemias to the cubic metre of water. The inference 

 is that, although the population is less dense near the bottom 

 than near the surface, yet on the whole the organisms may be- 

 said to be fairly uniformly distributed throughout the lake. My 

 observations are therefore in accordance with those of Eeighard 

 made during a biological examination of Lake St. Clair *. 



The fishings recorded above were made at 8 o'clock in the 

 morning, but others made at midnight (with strong moonlight) 

 and at midday showed that there does not seem to be any 

 extensive diurnal migration of the kind which Forel has 

 shown to be characteristic of the pelagic fauna of deep fresh- 

 water lakes, and which is also usual with many marine pelagic 

 animals. Indeed, during the day, owing to the shallowness of 

 Lake Urmi, the bottom is not much less brightly illuminated 

 than the surface, and a I'auna of negatively heliotropic nature 

 would search in vain for twilight in its inconsiderable depths. 

 I am of opinion, therefore, that 1200 Artemias per cubic metre 



* Bull. Michigan Fish Commission, 1894, No. 4. 



