56 Proceedings of the Royal Irish Academy. 



November, and December. In Loch Lubnaig it is fairly common in October, 

 but very rare in March. Thus the times of the maxima of Oedogonium in 

 these lakes differ from each other and from Tribonema in Lough Neagh. 



Wesenberg Lund records T. homhyciinun forma depauperata from all the 

 lakes examined by him in Denmark. It reached a very large maximum in 

 June and July in Esromso. 



No mention is made elsewhere of the occurrence of Tribonema hombycinum 

 in the plankton, but this is probably due to its very close resemblance to 

 Oedogonium. 



ZOOPLANKTON. 



ROTIFERA. 



Anurea. 



Anurea eochlearis var. tcda Grosse. — This is the most abundant rotifer in 

 the plankton of Lough Neagh. It is present throughout the year, and reaches 

 its maximum (142,500 in all catches) tovs^ards the end of July. The minimum 

 occurs early in February, and the numbers remain low until late in May. 

 Very much attention seems to have been given to this Eotifer, and hence its 

 biology is now rather well known. Its temporal variation in form has been 

 discussed elsewhere. Its length of life has been determined as about thirteen 

 days at 18° C. It forms part of the food of Abramis brama. 



Concerning distribution and times of of maxima in other lakes the follow- 

 ing may be said. Wesenberg Lund records the species from lakes, ponds, and 

 ditches in Denmark, and finds the maximum earlier than we, i.e. in May or 

 June. In the Baltic'lakes Apstein finds the species with a maximum at the 

 same time as ours — at the end of Jul}' in the Plouer See, but earlier in the 

 Dobersdorfer See. In the Scotch lakes Anurea eochlearis is of general 

 occurrence, and sometimes is so numerous as to render the water turbid. 

 A. eochlearis is a common feature of the arctic plankton, which is rich in 

 Eotifers ; and in short it well fills a place in that small community of species 

 designated by Wesenberg Lund as cosmopolitan. 



Anm-ea aculeaia Ehr. — This species occurs in the plankton of Lough Neagh 

 in small numbers only. It appears late in March, reaches its maximum in 

 July, and disappears completely towards the end of September. Wesenberg 

 Lund reports A. aculcata from most of the ponds and lakes of Denmark, where 

 it appears to be perennial. It reaches its maximum between April and June. 

 In the Baltic lakes, the Plouer See and Dobersdorfer See, this species reaches 

 its maximum in June, and is only present for part of the year. 



