166 



THE ANIMAL KINGDOM 



spores ;ire naked and nuisi be transniitted directly into the blood stream 

 ol the new host. 



Often, as in tlie malaria organisms, the formation of eggs and 

 sperm, fertiU/ation, and the formation of infective spores take place in 

 a different kind of host (e.g., a mos(piilo) from that in which trophozoite 

 stages are found. Such two-host systems and other phenomena associ- 

 ated with parasitism are discussed in Chapter 39. 



53. Reproduction in the Protozoa 



Asexual Reproduction. Asexual reproduction is found in all of 

 the protozoa. The nucleus divides mitotically, and the animal separates 

 into two complete organisms. The origin of the adchtional set of or- 

 ganelles differs from group to group. In Euglena (Fig. 8.14), the cen- 

 triole is the first to divide, then each centriole gives rise to a new basal 

 body. In the meantime the old pair of basal bodies move farther apart, 

 and the new pair come between them. The old fiagella separate, and 

 each new flagellum growing out from the new basal bodies fuses with an 

 old flagellum. The nucleus, which has gone through prophase and 

 metaphase, divides next. Separation into two individuals (Fig. 8.15) 

 begins anteriorly, and ends at the posterior tip. 



In Paramecium the division is transverse (Fig. 8.15). The old gullet 

 disappears and is replaced by two new gullets (in most other ciliates 



■Gullet 



Ba-Scd 

 bodies 



tCcnlrioles-k 



Nuclei 



Is;- ^ 



D 



Figure 8.14. Details of asexual reproduction in Euglena. In A the centriole has 

 already divided. R. Each centriole produces a new basal body and flagellum. The nucleus 

 is in prophase and the contractile vacuole is double. C, The old pair of flagellar roots 

 separate and fuse with the new roots. D, Mitosis proceeds and the gullet begins to divide. 



