6 PRACTICAL PARASITOLOGY 



swells up into a slimy colloid mass, is eminently suited to the purpose. 

 The gradual thickening of the medium does not appear to harm the 

 Protozoa, while its effect in retarding their movements not only 

 facilitates observation, but in some cases is the sole means by which 

 such is rendered possible. The best method is to put the seaweed 

 into a bowl with some water, and when it has swelled to the consist- 

 ency of a thick syrup, to introduce a small portion of the mass under 

 the edge of the cover-glass. Where the Infusoria or Flagellates are 

 not free-living but parasitic, the carragheen should be soaked in 

 normal saline solution, serum, &c., instead of water. Another way 

 is to add small portions of carragheen directly to the preparations, 

 but in this case undissolved particles of weed are liable to get in the 

 way and render observation difficult. 



This method of reducing the activity of Protozoa is one which 

 I invariably employ. I find that carragheen is more convenient to 

 use and offers better results than cherry gum, gum arabic or gelatine. 

 Statkewitsch * says that carragheen may be safely added to cultures 

 of Protozoa. If this is done, however, the pieces of carragheen 

 should first be washed in to 1 per cent, solution of bicarbonate of 

 sodium, they should be allowed to remain in the culture five to ten 

 days, and at the end of that time all undissolved particles should be 

 removed. After three to four weeks, the water in which the cultures 

 are kept must be carefully changed, and at the end of a further three 

 days fresh carragheen should be added. Paramoecia may be kept for 

 months like this, without in any way suffering from the viscidity of 

 the medium. The method possesses a certain analogy with that of the 

 cultivation of Amoeba upon solid media (see later) . 



Not the structure only, but the development of the Protozoa should 

 be studied as far as possible in the living organism, the various stages 

 presenting themselves to the student as a series of isolated but con- 

 secutive impressions. Care must be taken to prevent the medium in 

 which the Protozoa are to be examined and this applies particularly 

 to the parasitic Protozoa from becoming more concentrated by 

 evaporation. The specimens should be prepared as rapidly as pos- 

 sible, and the edge of the cover-glass should at once be painted round 

 with vaseline. Under these conditions the great majority of the 

 parasitic Protozoa will remain alive for about two hours. 



In addition to the usual microscopic methods, it is an advantage 

 to study living Protozoa by means of drop cultures. For these it is 

 necessary to use a glass slide which has been hollowed in the centre, 

 or (though this is less satisfactory) a glass slide on to which a glass 



1 P. Statkewitsch, " Zur Methodik der biologischen Untersuchungen iiber die 

 Protisten," Arch. f. Protistenkunde, vol. v, 1904, pp. 17-39. 



