124 INTESTINAL FLAGELLATES AND CILIATES 



cysted forms has very rarely been observed, and some writers 

 have even gone so far as to say that it does not occur. It is well 

 known that division into two individuals takes place after en- 

 cystment, and Wenyon has recently expressed the opinion that 

 if the division is completed before the cyst is expelled from the 

 body of the host, the cyst may burst and liberate the two animals, 



FiQ. 33. Giardia (or Lamblia) iniestinalis; A, side view (s. sucker-like depres- 

 sion); B, ventral view (par. b., parabasal bodies, n., nucleus); C, young cyst with 

 four nuclei; D, mature cyst containing two parasites; E, end view of young cyst; 

 F, parasite resting on epithelial cell. Figs. A-E, X 2000, after Wenyon; Fig. F, 

 X 1000, after Grassi and SchewiakoS. 



the cysts thus serving as a means of multipHcation. Kofoid 

 and Christiansen have recently succeeded in finding numerous 

 individuals of an allied parasite of the mouse, Giardia muris, 

 in process of division into two and also into four and eight indi- 

 viduals, both in the free and in the encysted state. That a simi- 

 lar process really occurs in the human parasite can hardly be 

 doubted, both from its similarity to the mouse parasite and from 

 the enormous numbers which may occur in an infected person 

 at one time. 



The free active parasites become motionless and die soon after 

 leaving the body of the host with the fseces, but encysted forms 

 (Fig. 33C, D and E) may retain their vitaUty for a very long time. 



