208 DAVID BRYCE ON THE 



they swim near the surface or deeper down according to the 

 level or zone in which the food organisms are most plentiful. 

 Another extensive series of species feed on the microscopic algae, 

 which grow more or less thickly upon the stems and leaves of 

 all kinds of water-plants, and, while readily swimming if dis- 

 turbed, seem to spend most of their lives in carefully seeking their 

 food where it grows, creeping all over the plants and nibbling 

 here and there. Others, again, are fond of temporarily perching 

 upon plants or confervoid threads, and feeding upon the organisms 

 carried to them by the currents set up by the play of their own 

 cilia. It is for the capture of rotifera of these various habits 

 that the ordinary use of the net is most advantageous. In 

 large ponds, where vegetation is frequently scanty, the net will 

 mainly capture species which habitually swim. In small, shallow 

 ponds with many and various water-plants, the second and third 

 groups may be expected to predominate. Such weedy ponds 

 frequently yield a much greater variety of species than the 

 larger ponds, especially if the net be roughly worked among the 

 vegetation to disturb those rotifers which may be creeping or 

 resting upon the plants. 



Judging from recent photographs of happy Quekett excursion 

 parties, the net mostly employed in such pond work is a quite 

 small, single net of about the shape of an ordinary funnel. I 

 have rarely used a net of this small type, and cannot therefore 

 speak positively as to the results obtainable by its use. But it 

 seems to me to be only useful for the capture of such rotifers as 

 swim quite near the surface, or for use in very shallow ponds. 



Now it has been shown by Dieffenbach * that in ponds of 

 moderate depth the swimming rotifers are throughout the day- 

 time in greatest abundance at a depth of about half a metre (say 

 20 inches) ; whilst during the night their distribution in the 

 water is practically uniform. It follows from this that the col- 

 lector who confines his use of the net in such ponds to the sur- 

 face of the water simply limits thereby the range of his captures. 

 The general object in using a net is not to capture a crowd of 



* H. Dieffenbach and R. Sachse, Biologische Untersuchungen an 

 Riidertieren in Teichgewassern, Internationale Revne der gesamten 

 Hydrobiologie und Hydrographies Leipzig. Biol. Suppl. 1U12. 



