DaKin and La'I'ARCHE — The Plankton of Lough Neagh. 69 



colonies occur in March and April, and that is just after the minimum. 

 Wesenberg Lund considers that the star-formation from the chain is an 

 adaptation from a littoral to a pelagic life. This may be the case ; but from 

 the above figures it would hardly appear that it is the buoyancy that determines 

 the change ; for we should then expect stars when the water reached the 

 highest temperature. 



The size of the cells of Asterionella varies a little more than in the case of 

 Tabellaria. They are as follows : — 



1911— Feb. 23, -318 ; Mar., -377 ; April, -361 ; May, -39 ; June, "35 ; 

 July, -398; Aug., -374; Sept., -34 ; Oct., -342 ; Nov.,-337 

 Dec, -323. 1912— Jan., -309 ; Feb., "318. 



The smallest individuals occur in the period December to February, that 

 is just below the minimum, with low numbers present, and the largest occur 

 irregularly between March and August, when the quantity present is great. 

 As a matter of fact, the largest cells measured occur in July and May, when 

 the number present was very great. 



Thus, though there is a slight difference between Asterionella and 

 Tabellaria, there is a general agreement in that the largest cells occur in the 

 summer months — a fact in complete opposition to the buoyancy theory 

 of Wesenberg Lund-Ostwald. Moreover, the large size seems to be 

 correlated with the large number present in the plankton and with small 

 colonies. 



Ceratium hirundinella. 



Many observers have studied variation in Geratiutn hirundinella, and their 

 original papers must suffice for a detailed study of this form. A few 

 observations have been made by us to determine whether variation takes 

 place in the Lough Neagh waters. 



C. hirundinella occurs as usual in Lough Neagh in two forms, the thi'ee- 

 horned and the four-horned. The three-horned foi'm has been stated by most 

 workers to be the commonest and to be the predominant form under arctic 

 conditions. The four-horned forms are supposed to occur chiefly in the 

 summer months. These facts have been recorded for German and Swiss 

 lakes. Some authors have stated that both forms are equally frequent at all 

 seasons. The variations recorded by "Wesenberg Lund seem to indicate that 

 the size increases from May to June, then decreases during July-August, 

 and then remains constant. 



Krause has made the most detailed observations on variation in 

 C hirundinella. This author agrees with the work of Ostwald, and describes 



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