36 



INVER TEBRA TE ZOO LOG Y 



peculiar flagellates. In the last-named genus are at least 100 

 nuclei and similar duplication of the groups of kinetic elements. 

 Trypanosomes (Fig. 23) occur as blood parasites in all classes 

 of vertebrates. In some instances, they are restricted to a single 

 vertebrate host species and in so far as has been observed these 

 are harmless to the host. Such is the condition which exists 



Blepharoplasi 



Cy^osiome. 

 Nucleus 



membrane 



Axoshjle 



Fig. 21. 



Fig. 22. 



Figs. 21 and 22. — Typical parasitic Zoomastigina showing, especially, the 

 organization of the neuromotor apparatus. 21, Giardia microti Kofoid and 

 Christiansen. Parabasals are two curved, dark bodies behind the axostyle. 

 {After Kofoid and Christiansen). 22, Trichomonas augusta Alexieff. {After 

 Kofoid and Swesy). 



between the rat host and its normal parasite Trypanosoma lewisi 

 which occurs in rats all over the earth. This species is trans- 

 mitted from one rat to another by the rat flea. The trypano- 

 somes are taken into the stomach of the flea when blood is sucked 

 from an infected rat. The trypanosomes penetrate the tissue 

 cells of the flea and undergo multiple fission. After considerable 

 change in form they penetrate into the rectum of the flea and are 

 passed out with the feces. Reentrance into the rat is not by the 

 bite of the flea but occurs when the rat licks its fur. 



