48 H. WHITEHEAD ON BRITISH FRESHWATER RHABDOCOELIDA 



prefer still or gently flowing water to rapid streams. One 

 species, Prorhynchus stag7ialis, is sometimes found on moist earth. 

 Many of the aquatic forms are free swimmers, and may be 

 captured in the net in the same way as rotifers and water-fleas ; 

 others live in mud. In the latter case it is best to pour a little 

 of the mud into a glass tank containing clear water, and to 

 remove any Rhabdocoels by means of a pipette. They should 

 be examined in a live box, and it will be found that a slight 

 pressure is necessary to ensure making out their internal 

 structure. They are very difficult to prepare in a satisfactory 

 manner as permanent objects, and the writer has made numer- 

 ous experiments with a view to narcotising them, but with 

 little success. Eucaine, chloroform, ether and alcohol are of no 

 use. The difficulty seems to lie in the fact that the rhabdites 

 are discharged as soon as the animal is irritated, and these, of 

 course, produce quantities of mucus. Moreover, the epidermal 

 cells get destroyed during the process. The only satisfactory 

 method of killing seems to be by means of some hardening re- 

 agent, like corrosive sublimate solution, which takes effect before 

 the mucus and rhabdites can be discharged. The following well- 

 known method is the best. The specimen is placed in a watch- 

 glass with a little water, the bulk of which is withdrawn by a 

 pipette. A drop of Lang's Fluid is then delivered from a pipette 

 on the side of the watch-glass and is allowed to run over the 

 animal. Death is almost instantaneous, and but little shrinkage 

 takes place. Even with this method the writer has not yet 

 succeeded in killing species of Mesostoma without disruption. 

 After remaining in Lang's Fluid from ten to fifteen minutes, 

 the specimens are removed to 45-per-cent. spirit. They are 

 afterwards passed through alcohol of increasing strength, stained 

 with borax-carmine and mounted in Canada balsam in the usual 

 way. 



Some of the Rhabdocoels appear to be entirely vegetarian in 

 diet, and consume desmids, diatoms and unicellular algae. In 

 fact, care is sometimes necessary to distinguish the food from 

 the zoochlorellae. The latter, however, never occur in the gut. 

 The majority of species take animal food, which consists of water- 

 fleas, small worms, etc. 



We may now consider a few typical species which have been 

 taken by the writer in the neighbourhood of London. 



