OBSERVATIONS ON THE PLANKTON OF LOOE POOL. 331 



Polygonum amphibium, Potamogeton perfoliatus, P. pusillus, and 

 Elatine liexandra. To these may be added Niti'lla hyalina, a plant 

 new to Britain This addition to the fauna of the United 

 Kingdom was discovered in Penrose Creek during August, 1898, 

 by the Eev. G, E. Bullock- Webster. A large bed of rushes 

 occTipies the delta at the junction of the river with the pool. 

 Isolated specimens of the same plant are scattered along the 

 shores of the lake at irregular intervals. A small clump of reeds 

 is to be found in Penrose Creek ; and, close by, a large bed of 

 water-lilies, recently introduced, appears to be in a flourishing 

 condition. 



Contrary to my expectations, no species of Eotifera belong- 

 ing to the sedentary orders of PlosculariadtB and Melicertadse 

 have been found living on any of the aquatic plants ; although 

 several species of free-swimming forms are recorded in my lists. 



That the water in the pool is not pure is clearly shewn by 

 the absence of the sessile-eyed crustacean Gamarus pulex. It has 

 long been known to zoologists that the normal habitat of this 

 species is in pure-water lakes, where it affords excellent food for 

 trout, and that when transferred to impure water it quickly dies. 



An interesting little fresh-water mollusk, Ancula cristata, is 

 fairly abundant, adhering to the under- surfaces of the large flat 

 stones which fringe the lake along its western edge. Speaking 

 generally, I have not found the littoral fauna to be at all rich in 

 any portion of the lake ; indeed it is very scarce. 



Perhaps the most striking incident observed during the period 

 over which these observations extend was the sudden decline in 

 the plankton towards the end of 1899. On examining a series of 

 gatherings made on the 30th of November, the quantity of the 

 floating fauna was, considering the tiiae of year, above the 

 average ; this being in a great measure due to the mildness of the 

 season. About a month later, viz. on the 29th of December, on 

 working the tow-net in the usual manner, few living forms could 

 be found in the tin receiver at the end of the tow-net after each 

 haul. I believe this sudden decrease in the plankton to be due 

 to the heavy rain-fall which washed the free -swimming forms into 

 the sea. Sev^eral ponds in my neighbourhood during this time 

 were found to contain plenty of free swimming life, although 



