PROTOZOA 



either are not apparent in the preserved and coloured specimen, or 

 are, at any rate, seen to much better advantage in the living object. 

 It is true that the examination of living Protozoa is sometimes a 

 matter of difficulty as, for instance, where the individuals are very 

 much crowded together ; where the plasm is exceptionally rich in 

 light-refracting contents ; or where opaque cyst- or shell-formations 

 are present. But, if the correct diaphragm is used with the micro- 

 scope, it is generally possible to obtain a view of the greater portion 

 of the internal structure. Wherever fresh material is forthcoming, 

 the living organism should always be examined first. 



It is, however, expedient, and in certain kinds of scientific work it 

 is indeed absolutely essential, to combine the two methods. Coloured 

 specimens should be prepared and examined, after which the fresh 

 material should again be studied. The knowledge gained by examina- 

 tion of the coloured specimens will show the living organism in a new 

 light, and reveal the true significance of many details which at first 

 escaped attention. 



Wherever possible, parasitic Protozoa should be examined in their 

 natural medium, i.e., intestinal parasites in undiluted faeces or intes- 

 tinal secretion ; blood parasites in blood ; and tissue parasites in 

 lymph. 



Should it be necessary to filter the liquid in which isolated parasites 

 are to be examined, and especially where this liquid is obtainable only 

 in very minute quantities (as the intestinal secretion of insects), the 

 following method should be adopted : The preparation from which the 

 liquid is to be obtained (in this instance a section of the intestine) is 

 placed upon a morsel of filter paper on a cover-glass. The fluid rapidly 

 niters through the paper and appears as a thin film upon the glass, to 

 which the parasites to be examined are then transferred. Should the 

 natural fluid be insufficient in quantity and this is very frequently 

 the case when examining the parasites of small insects, such as mos- 

 quitoes it may be increased by the addition of normal saline solution. 

 But it must be borne in mind that the addition of normal saline will 

 shorten the life of the parasites. For this reason, it is sometimes 

 better to increase the quantity of natural liquid by supplies drawn 

 from the bodies of individuals of the same species as the host, the 

 method of filtering above described being employed wherever necessary. 



When studying the more active of the Protozoa (Flagellates, 

 Ciliates) it will frequently be found necessary to restrict their inotility 

 to a certain extent. This is best done by increasing the viscosity of 

 the medium in which they are to be examined by adding some colloid 

 matter. The dried and bleached seaweed sold by druggists under the 

 name of carragheen (it consists mainly of Chondrus crispus with a 

 small quantity of Gigartina mamillosa), which, when soaked in water 



