INTRODUCTION 27 



know too little on this subject to generalize; where animals are 

 segregated the opportunities for parasitism are enhanced while some 

 climatic conditions are more ad^•antageolls than others for the 

 spreading of germs. Thus the blood-dwelling parasites are more 

 common in the tropics than elsewhere, the biological conditions 

 favorable to the intermediate transmitting hosts being largely 

 responsible for their numbers and variety. 



GENERAL ORGANIZATION OF THE PROTOZOAN BODY. 



Although Protozoa belong unquestionably to the microscopic 

 world their sizes va^^' within wide limits. Some are large enough 

 to be picked up with forceps {Porospora gigautea, up to 16 mm.) 

 and many of the larger ciliates are easily visible to the unaided eye 

 {Bursaria tnmcatella, Spirostoinum avibiguum) while many smaller 

 types can be seen by the trained eye as mere white specks which, 

 in some cases, may be identified by their characteristic movements 

 {e. g., Paramecium, Frontonia, DUeptus, AmphUeptus, Loxophyllum, 

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

 visible even with the most powerful lenses of the microscope. 

 From 8 to 16 such forms have ample room for existence in a red 

 blood corpuscle (Bahesia canis), or 200 to 300 may live simulta- 

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

 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 2^^77-0 of an inch. Thus Leish- 

 mania donovani measures from 2^t to 4/z, Paramecium caudatuni 

 upward of 200^, Bursaria truncafeUa, 1500/i, 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 caudafum may 

 measure from 175// to 250 At when fully grown and similar variations 

 are characteristic of all species. Environmental conditions, espe- 

 cially food conditions, are frequently responsible for changes in 

 size and character of a species, often rendering them difficult to 

 recognize and affording tempting opportunities for swelling the list 

 of synonyms by new names for the abnormal forms. Thus Dileptus 

 anser when starved has a very different size and character from the 

 normal form (Fig. 6). Again, different normal stages in the life 

 history of a given species are not infrequently mistaken for different 

 species, largely because of difference in size. Thus Vroleptus 

 mohilis (see Fig. 1), in its adult vegetative condition, measures 

 about 150yu, but immediately after conjugation not only is it reduced 

 by one-third in size, but its internal structure is entirely different 

 from that of the usual form, while during the period of old age it 



