t)AKiN AND Latarche — The Plankton of Lough Neagh. 49 



many marine species, there are only two fresh-water species ; and one of 

 these, G. comutum Clap, and Lachm., is a pond-form and does not occur in 

 LoiTgh Neagh. Ceratium is apparently absent in January and February 

 alone. We say apparently because it was only after the investigation of a 

 large quantity of December plankton that some odd individuals were found. 

 It is very probable therefore that a few isolated individuals occur right 

 through the year. In many lakes C. liirundinella is known to be altogether 

 absent in the winter months. Wesenberg Lund states that it begins to 

 appear in May in the Danish lakes. We have found a few resting cysts, 

 and probably in this form as well as in isolated individuals the genus 

 survives through the winter. It appears in definite numbers in the Lough 

 Neagh plankton in March, but it is then very rare. A gradual increase 

 takes place from this time until its maximum is reached in August. At this 

 time it is one of the organisms occurring in greatest numbers in the plankton. 

 Its maximum coincided with the maximum of Tabellaria and Asterionella; 

 otherwise it would have been the most numerous organism in the plankton. 

 As it is, it practically comes second. Both three-horned and four-horned 

 individuals occur ; but the four-horned is by far the most common when the 

 maximum is reached. 



For details of the form- variation, reference should be made to p. 69, where 

 this has been discussed. 



After the maximum in August, the number drops very suddenly (from 

 2,086,500 individuals in all catches to 233,710 on September 8th). There are 

 about as many present in October as in June. Thus G. hiruiidinella has one 

 very definite maximum, and this agrees with the marine species in occurring 

 in the warmer months. 



Apstein records G. hirundindla in the Baltic lakes, where it has one 

 maximum in the summer months and is absent during the winter. Messrs. 

 West record G. hincndinella from the Scottish lakes, Orkneys, and Shetlands, 

 west and south-west Ireland, the Welsh lakes, the English lakes, Lough 

 Neagh, and Lough Beg. 



In Ennerdale Water it is common in September and October, when it 

 reaches its maximum. There is a smaller maximum during June and July. 

 The species is absent from December to April. It does not occur in 

 Wastwater at all. G. liirundinella is present in Loch Lomond, but not 

 common. The maximum for this lake is attained in September. The species 

 is absent altogether from November to May. 



G. hiruiidinella does not appear to be present at all in Loch Katrine, 

 whilst in Lough Earn it is only recorded for August, and even then it is rare. 

 Messrs. West do not find it in Lough Lubnaig. 



