130 



THE RISE OF ANIMAL LIFE 



Fig. 7-24. Life cycle of the dysentery amoeba (Endameba histolyfica). Infective cysts are carried on the hands of 

 food handlers and thus transmitted directly to uninfected people on uncooked food. The trophozoites emerge 

 from the cysts in the intestine where they multiply and feed on the tissues and blood of the host, thus producing 

 serious illness. 



tarded areas of this country show an infec- 

 tion rate as high as 23 per cent, although 

 for the nation at large it is around 5-10 per 

 cent. Varying degrees of success have been 

 achieved with a great many substances in 

 attempts to combat the disease. Of the anti- 

 biotics, penicillin and Chloromycetin have 

 recently proven the most effective in exper- 

 imental animals. 



Class Sporozoa 



Members of this group possess no appar- 

 ent means of locomotion and thev lack con- 



tractile vacuoles. They reproduce asexually 

 by multiple fission, and they are all para- 

 sites. At some stage in their complex life 

 cycle they produce sex cells, macro- and 

 microgametes, which fuse in the formation 

 of zygotes. The asexual stage produces the 

 sexual stage, which in turn gives rise to the 

 asexual phase, thus completing the cycle. 

 This alternation of sexual and asexual gen- 

 erations is spoken of as metagenesis. We 

 will have occasion to study this biological 

 phenomenon in metazoan animals a little 

 later. One or both of the asexual and sexual 



