Dakin and Latahche — The Plankton of Lough Neagh. 71 



states that the long spine is characteristic of winter forms, and is reduced or 

 disappears in the summer. Wesenberg Lund states, on the other hand, that 

 the posterior spine is longer in summer than in winter. It is also stated by 

 Wesenberg Lund that seasonal variation is not so well marked, or is lacking 

 altogether in arctic-alpine lakes and in the pelagic regions of larger lakes. 



We have ourselves found that seasonal variation is very definite in its 

 course. The tecta series is most abundant at all times ; but we have seldom 

 found individuals with no posterior spine, though it may be slightly reduced 

 in length. 



Anurea cochlearis (total length). 



1911— Feb. 28, 1-215; March, 1'22 ; April, 1-33; May, 1-275 ; 



June, 1-258 ; July, 1-285 ; August, 1-033 ; Sept., -98S : 



Oct., 1-02; Nov., 1-048; Dec, 1-075. 



1912— Jan. MI; Feb. 1-148. 



The series of averages given above are very remarkable indeed, in the 

 fact that the curve for length is extremely irregular. There can be no 

 shadow of doubt from the above figures that the size of Anurea varies in an 

 extremely irregular manner. Moreover, there is a decrease in size with rise 

 in temperature after April, and after September an increase in size with fall 

 of temperature. This is in perfect agreement with the Wesenberg Lund- 

 Ostwald theory ; but there is no reason why the increase in size should 

 continue regularly until the month of April. As a matter of fact, the Anurea 

 agree better with the Ostwald viscosity theory than any other species of zoo- 

 or phytoplankton ; and yet it is in the Eotifera that experiments have shown 

 other factors are determinants, because they happen to run a parallel course 

 to form-variation. The temperature factor might act in other and more 

 direct ways. 



The only spineless form found occurred in September, when the smallest 

 individuals were observed. On the Ostwald hypothesis we should have 

 expected small individuals vAtli long spines in the summer months. This 

 reduction in size of the spine agrees with Lauterborn's own researches, and 

 gives no explanation of Wesenberg Lund's statement that the spines were 

 longer in summer forms. 



So far as the Crustacea are concerned, we have had no time as yet to make 

 detailed series of measurements, though from the observation of catches we 

 know that the definite changes in the shape of the head take place. 



