THE FRESH- WATER LOCHS OF SCOTLAND. 141 



occasion do so. Asterionella, Xotholca longispina, Asplanchna 

 priodonta, Ceratium hiriitidhiella, and even on one occasion the rather 

 uncommon Rotifer Dinocharis Collinsii, have been observed to form a 

 " Wasserblut " in the shallower lochs. 



The abundance of certain species in a loch on a single visit may be 

 exceptional or temporary ; the small lochs may vary greatly at different 

 seasons. It is believed that, except for the seasonal appearance of 

 certain species which are known to live for only a few months of the 

 year, a loch is pretty uniform in character throughout the year. This 

 is known to be the case with the large lochs and with some small ones. 



The points to which attention will be called in reviewing the biology 

 of the Tay lochs will be the abundance or scarcity of life on the 

 whole; the preponderance of one or a few species in each loch; the 

 abundance of an animal or plant that is usually scarce; the absence 

 or scarcity of some very common species. 



The lochs of the valley of the Earn differ much in size and physical 

 conditions, so that they might be expected also to differ much in their 

 biology. There is one great lake, Loch Earn, two hill lochs, Turret 

 and Uaiue, the latter at a great elevation, one deep but stagnant pond, 

 and one shallow artificial dam. 



Loch Earn. The only abundant organism was Diaptomus gracilis, 

 which was bright red in colour. There was almost no life at the 

 surface, the Diaptomu* being in myriads at a depth of 40 or 50 feet. 

 The loch was rather remarkable for the scarcity of common lacustrine 

 species. Bythotrephes was somewhat frequent; Polyphemus, Cyclops 

 strenuus, and Bosm-ina obtusirostris were present, but not plentiful. 

 Daphnia was very rare, only one example being seen. Smaller 

 organisms were almost entirely absent, except for a few examples of 

 the two commonest pelagic Rotifers : Anurcea cochleare and Xotholca 

 longispina, and some unicellular Algae. 



Loch Turret. This was one of the lochs where Holopedium filled 

 the net with a slimy mass, and rendered it difficult to catch anything 

 else. Diaptomus y radii* , Daphnia (typical D. lacustris), Asterionella, 

 Pfridinhun tahulatum were noted. 



Lochan Uaine. This little shallow tarn, in a corrie at a considerable 

 elevation, had nothing remarkable in its pelagic life. Diaphanosoma 

 brachyuruin was most numerous ; Diaptotnus yradlis, of a brown colour, 

 and Polyphemus were common. Only a few examples of Daphnia 

 larustrts and Holopedium were seen. 



Loch ^[<>n:ie^clird (or Ochtertyre). This loch, though fairly deep, 

 was almost stagnant at the time it was visited. As might be expected 

 from this and from the very high surface temperature, life was abundant 

 and varied. The collection was green from the abundance of Volvor. 

 Bosmina cor nut a, Daphnia lacustris, Diaptomus yracilis (of a brown 



