Stkphkns — The Fresh-water Sponges of Ireland. 217 



Speaking generally, as long as there is a suitable substratum on which 

 sponges can establish themselves, it is but seldom that one will return 

 empty-handed from a search in any lake or river. But, as is always the case 

 when this group of animals is concerned, the general rule has exceptions. 



As far as my experience goes, sponges are not found in mountain streams 

 in Ireland, unless there is a lake, however small, in the course of the stream. 

 They do nob occur in the streams flowing into the lake, but are to be found 

 in tlie out-flowing stream or streams at a point immediately below the lake. 

 Even when the sponges are few in number and small in size in the lake 

 itself, just below it they often spread out in masses over the under surface 

 of the larger stones, and if these upper stones are removed they are to be 

 seen carpeting a lower layer of stones in the bed of the stream. The species 

 found in such situations are Hctcromcijenia Ri/deri and, more rarely, Spont/illa 

 lacustris. If the mountain stream is small, the sponges appear to die out 

 aarain further down its course, or at least they do not occur in sucli 

 abundance. 1 



In lower-lying country sponges are found in the larger streams and 

 rivers whose course does not pass through a lake. In this case they do not 

 appear to grow very near its source. Probably it is owing to an insufficient 

 food supply in a river near its source, and in a mountain stream, unless tliere 

 is a lake in its course to act as a sort of reservoir, that sponges are not found 

 in these situations. 



Sometimes sponges cannot be found when shore-collecting in lakes which 

 appear to be eminently suited to their growth, possessing, for example, clean 

 stone-strewn beaches and clear water. No reason can be assigned to account 

 for their absence. But it is possible in some cases after a very dry 

 summer, when the water-level is unusually low, and the sponges are killed 

 along a wide strip of shore, which is thus exposed, that it may take some 

 time for them to reach again, at least in their former numbers, to their usual 

 level. For instance, Lougli Gill was examined for sponges in July, 1914, 

 and they were found growing in abundance on the metamorpliic rocks in 

 certain places, and in smaller numbers on the limestone along other parts of 

 the shore (43j. Tlie water was exceptionally low that year, and dried 

 sponges were seen on the stones well above the watei'-level. A visit lo 



' In this conne.'cion it is interesting to quote a statement of Dr. .■Vnnaiidale's in a 

 paper (8) received after the foregoing was written, in which he discusses the occurrence 

 of sponges in mountain streams in ludia. He writes: " I liave not yet found any sponge 

 in a small mountain torrent such as those at Khandalla, in whicli food is probably 

 deficient ; but when these streams are dammed to form ponds in which aquatic 

 vegetation grows up, sponges soon make their appearance." 



