INTRODUCTION 



27 



to be picked up with forceps (Porospora gigantea, a gregarine, up to 

 16 mm.) and many of the larger ciliates are easily visible to the 

 unaided eye (Bursaria truncatella, Spirostomum ambiguum) while 

 many smaller types can be seen by the trained eye as mere white 

 specks which, in some cases, may be identified by their characteris- 

 tic movements (e. g., Paramecium, Frontonia, Dileptus, Amphileptus, 

 Loxophyllum, etc.). At the other extreme in size are types which 

 are barely visible even with the most powerful lenses of the micro- 

 scope. From 8 to 16 such forms have ample room for existence in 

 a red blood corpuscle (Babesia canis), or 200 to 300 may live simulta- 

 neously in a single infected liver or spleen cell of man (Leishmania 





B 



k 



Fig. G. — Dileptus gigas, two sister cells. A, normal individual; B, individual starved 

 for several days. (From Calkins.) 



donovani). Between these two extremes of size lie the majority of 

 Protozoa. Their measurements are usually expressed in terms of 

 " microns " or thousandth parts of a millimeter which are represented 

 by the symbol n, each micron being 2t|-oo- °^ an ^ ncn - Thus Leish- 

 mania donovani measures from 2 n to 4 /x, Paramecium caudatum 

 upward of 200 //, Bursaria truncatella, 1500 /x, etc. 



The same species frequently shows remarkable variations in size 

 due to environmental conditions or to different stages in the life 

 history. Thus normal specimens of Paramecium caudatum may 

 measure from 175 /j, to 250 /x when fully grown and similar variations 

 are characteristic of all species. Environmental factors, especially 



