PHYLUM PROTOZOA. ONE-CELLED PARASITES 



51 



A living or preserved earthworm should be 

 pinned down and a slit made in the body 

 wall from about the tenth to the fifteenth 

 segment; the whitish bodies that extrude 

 are the seminal vesicles. Parts of these should 

 be pinched off with forceps and teased out 

 well with dissection needles on a slide, in 

 a drop of 0.7 per cent table salt (NaCl) 

 solution. A cover glass should be placed on 



Young trophozoite 



the preparation, which should then be ex- 

 amined under the microscope. 



The life cycle of Monocystis is briefly out- 

 lined as follows (Fig. 23). The spores are 

 taken into the earthworm's digestive tract 

 where the sporozoites are set free. Each 

 sporozoite penetrates a bundle of develop- 

 ing sperm cells in the testis of the earth- 

 worm and is then termed a trophozoite. 



Sporozoite enters developing 

 sperm cells in testes 



Trophozoite grows and 

 migrates to seminal vesicle 



Cyst wa 



Spores containing sporozoites 

 escape and are eaten by 

 another worm 



Trophozoites associate 

 in pairs and form a 

 cyst wo I 



Spores 



Gametes 

 Zygotes 



Zygotes secrete a spore 

 wall and divide to form 

 8 sporozoites in each spore 



Gametes are produced 



Gametes fuse (fertilization) 

 to form zygotes 



Figure 23. Life cycle of Monocystis, a sporozoan that lives in the common earthworm. All 

 highly magnified but not drawn to scale. 



Here it lives at the expense of the cells 

 among which it lies. The sperms of the 

 earthworm, which are deprived of nourish- 

 ment by the parasite, slowly shrivel up, be- 



coming tiny filaments on the surface of the 

 trophozoite, making it resemble a ciliated 

 organism. The trophozoite grows and then 

 migrates to a seminal vesicle. Here two 



