THE ROTIFER A OF THE SCOTTISH LOCHS. 185 



Note on the Rotifera of Ponds, as compared with Lakes. 



Neither physically nor biologically can any hard and fast line be drawn between 

 lochs and ponds. In making the bathymetrical survey, the practice was to examine 

 any body of water on which a boat was found, or could easily be placed, omitting as a 

 rule those of less than quarter of a mile in greatest diameter. 



Some lochans of less than quarter of a mile in length — for example, Lochan Dubh at 

 Lochailort — were of such depth that the temperature and the biological phenomena 

 showed a correspondence with our greatest lakes, rather than with shallow lakes or 

 ponds. On the other hand, some very considerable lochs were so shallow that the tem- 

 perature had the extreme range found in small ponds, and the biology corresponded. 



The foregoing list of Rotifers is restricted to species found in lochs which were 

 bathymetrically surveyed. When practicable, we also examined ponds adjoining the lochs 

 for purposes of comparison, and periodical collections are being made from certain small 

 ponds, in order to compare the annual cycle of changes with that which occurs in lochs. 

 This investigation is still incomplete, and will be dealt with when finished A full 

 account of the Rotifera of ponds would be too extensive to be included here. At present 

 it is only intended to contrast the relative frequency of the species in our list in lakes 

 and ponds, and to describe a free-swimming Bdelloid which came to light in the course 

 of these researches. 



All the Rhizota in the list, except the species of Conochilus, Floscularia pelagica, 

 and F. mutabilis, are commoner in ponds, and are not very commonly found in lakes. 

 Even the powerful swimmer, Pseudcecistes rotifer, prefers ponds and ditches. Of the 

 Bdelloida, the various genera are different in their habits. On the whole, the oviparous 

 kinds are more at home in lakes, the viviparous in ponds, but there are exceptions. Most 

 of the species of Philodina which we have recorded are very much at home in lakes, 

 and several of them, with the related Microdina, are the most characteristic of lake- 

 margin forms. P. citrina, P. acuticornis, and the two viviparous species are pond- 

 dwellers. 



The genus Rotifer is on the whole rare in lochs. I have found no species common 

 in lochs except the parasitic R. socialis ; R. macroceros is next in frequency, the 

 others rare. 



In ponds adjoining Loch Ness we found R. vulgaris, R. citrinus, R, tardus, 

 R. lonyirostris, R. macrurus (which is not in our list as a lake species) all common 

 and abundant. R. neptunius is frequent, and R. trisecatus not very rare in ponds. 

 R. macrurus is most at home in bog-pools, and R. longirostris among dirty moss. 



Many of the Callidinse are ubiquitous — equally at home in ponds, lochs, and else- 

 where. None are particularly characteristic of lake-margins, but C. muricata, 

 'C. crucicornis, and C. incrassata (not yet found in lochs) are true pond species. 



The pellet-making Callidinse, though well represented in our list, are with few 

 .exceptions properly peat-bog species. C. elegans (?), C. pusilla, and C. longiceps are 



