82 Proceedipgs of the Rot/al Irish Academtf. 



and others hare made their catches under a peculiar condition which prevails 

 for exactly half a year only — that of day, and have apparently considered 

 that the conditions of night and darkness make no difference or only a s%ht 

 change. It is well known, of course, to all marine biologists that many 

 plankton forms — Sehizopods, Cumacea, and Polychaete larvae — rise towards 

 the surface at night. Many, if not most, of these forms occur, however, in the 

 plankton by day. The difference between day and night surface-plankton in 

 the Ii-ish Sea, for example, is probably in the relative numbers of forms present 

 both by day and night. In Lough Xeagh, on the other hand, a year's plankton 

 catches by day gave no species larger than Cladocera. The fii-st midnight 

 catch was a striking contrast. It was a calm night with a full moon hidden 

 completely by clouds. The sky gave a kind of dull yellow-green light which 

 just served to make out the outlines of brightly painted or white articles in 

 the boat. A surface haul of a few minutes only was taken, and on the net 

 being emptied in a la;rge vessel of water the latter was simply boiling with 

 Mysis relicta actively swimming. The light from the electric torch drove them 

 all to the bottom of the vessel in a few seconds, where they formed almost a 

 solid mass, so numerous were they. Xo marine catches taken at night ever 

 have shown such startling contrasts as the day and night catches on Lough 

 Neagh. The water was 40 feet de«p where these Mysis were taken at the 

 surface. An hour and a half at the same place next morning gave not a 

 single indi\idual in the water ! 



On many an occasion whilst in Belfast and district, 1 was informed that 

 Loxigh Xeagh was once an arm of the sea, cut off, and gradually converted into 

 fresh water. Theproof of this was the presence of ifysis relicta: and apparently 

 this was considered quite sufficient by many naturalists to indicate the origin 

 of the lake. 



Before discussing this question, we must refer to the origin of the fresh- 

 water plankton. On the whole, the fresh- water plankton differs greatly from 

 the marine plankton. Numerous details were constantly Ijefore our eyes — 

 especially when we compared our Irish Sea plankton with that of Lough 

 Xeagh. Probably but few fresh- water plankton forms are direct immigrations 

 from the sea, and we must look for some other origin. TVesenbei-g Lund has 

 treated this subject very carefully, and considers it extremely probable that 

 we must look to the littoral and bottom fauna and flora for the origin of 

 oirr pelagic organisms. Whether we take this as correct or not, we must 

 look to the sea for the ultimate origin of the lininoplankton, and the 

 question of the migration into fresh water must remain for some time longer 

 unsettled There is, however, a stnktng resemblance between many 

 alpine and arctic plankton forms: and other details, too, have led many 



