THE PROTOZOA 



Fig. 8.14. Stages in the life cycle 

 of Monocystis, a sporozoan inhabit- 

 ing the seminal vesicles of earth- 

 worms. .4, infective stage, or 

 spore, containing eight sporozoites. 

 /?, spore in cross section. C and 

 D, sporozoites after release from 

 spore case. E and I'\ trophozoites 

 growing within sperm spheres of 

 the earthworm. G, mature troph- 

 ozoite surrounded by remnants of 

 the sperm sphere; the tails of the 

 spermatozoa resemble cilia. //, 

 two trophozoites, free of debris, 

 uniting as gametocytes. / and /, 

 encyst ment of gametocytes and 

 multiple division (.sporulation) to 

 form gametes. A, fusion of pairs 

 of gametes to form zygotes; the 

 residual cytoplasm of the gameto- 

 cytes remains in the center of the 

 cvst. L, young spores contained 

 in the large cyst; within its spindle- 

 shaped spore case, each spore un- 

 dergoes multiple fi.ssion to produce 

 eight sporozoites, arranged as in A 

 and B, and these mav then be 

 transferred to another worm. 



D 



an intracellular parasite and later lies free in the fluid of the seminal vesicle. 

 Presumably, food is absorbed through the cell membrane from the surround- 

 ing medium, and metabolic wastes are eliminated by diffusion. An abundant 

 reserve of nutrients is stored in the cytoplasm and is utilized during encysted 

 and gamete-forming stages. The life cycle, shown diagrammatically in Figure 

 8.14, contains a stage in which rapid, successive divisions produce a large 

 number of spores. This type of proliferative division, termed multiple fission, 

 is a characteristic feature of the life cycles of all sporozoans. 



Other representative Sporozoa may be listed as follows: species of the genus 

 Gregarina, common in the digestive tracts of insects; the coccldians, which 

 parasitize vertebrates and invertebrates and are exemplified by the' genus 

 Eimena; and the malaria parasites. 



245 



