COLLECTION OF BDELLOID AND OTHER ROTIFERA. 211 



this purpose what became known after him as the " Hardy flat 

 bottle," which can be easily made in the manner described later. 



The aquatic plants most productive are those with finely 

 divided leaves, such as the various water Ranunculi, Cerato- 

 phyllum, Myriophyllum, etc. The ubiquitous Elodea canadensis 

 (better known perhaps as Anacharis) is also a favourite plant, 

 and the humble Duckweed Letnna minor should not be despised, 

 as it is much favoured by the plant-loving rotifers, and some 

 of the very earliest discoveries of such forms were made from 

 the examination of the pendant roots of this floating species. 

 The ivy-leaved Duckweed, Lemna trisulca, I have not found to 

 be so useful. 



The portions of water-plants brought home can be kept alive 

 in glass jars or aquaria for periods varying according to the 

 plants themselves, some enduring under such conditions for 

 'months, others only a few days. It is desirable to have at the 

 bottom of such receptacles a moderately thin layer of muddy 

 sand to provide food for the small protozoa which in their turn 

 form the principal food of many of the Rhizotan Rotifera. 



Of those species which habitually feed on the broken-down 

 vegetable substances at the bottom of the pond, ordinary pond 

 work with the net will only capture stray individuals. By the 

 use of the dipping bottle a better selection can frequently be 

 secured, either by taking home a single bottleful, or by passing 

 several bottlefuls through the net and taking the results home. 



These bottom-feeding species are usually accompanied by 

 several species of the Gastrotricha, which are mostly of the same 

 habit. Dr. Voigt, a very successful collector of Gastrotricha, 

 during two years spent at the Plon Biological Station, made use 

 of the following more elaborate method : 



A narrow-necked bottle of suitable capacity had its lower end 

 fixed into a canister partly filled with lead. To the canister 

 were attached two cords which were brought together in a ring 

 above the neck of the bottle. The whole was knotted to a line 

 and was then ready for use. The adjustment of the various 

 cords was so arranged that when the bottle was dropped into 

 the water at the desired spot, the mouth could be caused to lie 

 close above the muddy bottom, when the gradual filling of the 



