218 FRESH-WATER BIOLOGY 



The ooze at the bottom of ponds or lakes is the habitat of nu- 

 merous shell-bearing as well as naked forms. Others, like the 

 Heliozoa, are commonly found among algae, diatoms, or mosses and 

 may be collected with these plants. Inactive or encysted forms 

 gathered during cold seasons of the year wil become active on 

 being placed in a warm laboratory. Shallow aquaria are best 

 adapted for preserving quantities of living Sarcodina. For those 

 species which require it, the water may be kept fresh by algae or 

 other aquatic plants, but for many forms the water may be allowed 

 to become stagnant, replenishing it only as evaporation takes place. 

 The Sarcodina may be studied with a considerable degree of satis- 

 faction, as it is possible to keep them under observation for an in- 

 definite time, owing to their slow movements. For detailed study 

 a good compound microscope including an oil immersion lens is 

 necessary. Concave microscopic slides on which living forms may 

 be isolated and retained for extended observation are useful. 

 Methylenblue, used as an intramtam stain, is successful in render- 

 ing the nuclear elements visible, especially in species without shells 

 or with transparent envelopes. 



When permanent mounts are desired the following method, rec- 

 ommended by Benedict in the Journal of Applied Microscopy, 

 Vol. VI, p. 2647, mav De employed: "Smear a glass slide with 

 albumen fixative, as in preparing for the mounting of paraffin sec- 

 tions. Then place on the surface of the film of fixative a drop or 

 two of water containing the forms which it is desired to stain. 

 Let nearly all the water evaporate by exposure to the air of the 

 room until only the film of fixative remains moist. The slide can 

 now be immersed in Gilson's or any other fixing reagent and then 

 passed through the alcohols, stains, etc., in the same way that 

 mounted sections are handled." 



The above method is recommended for other Protozoa as well as 

 for Sarcodina. As a rapid fixing agent, the fumes of osmic acid 

 have been found satisfactory. By careful manipulation of fine 

 dissecting needles under the microscope, the shells of many forms 

 may be isolated, arranged as desired and, when dried on the slide, 

 permanently mounted in balsam. 



