218 



THE ANIMAL KINGDOM 



ADULT 



Procercoid 



Ccrcoid 



Figure 11.15. Life cycle of a tapeworm. All of the stages are those of the fish tape- 

 worm, Diphyllobotlniiiin latum. (Modified from Hyman, after Rosen.) 



In tape^V'Orms with a three-host cycle the oncosphere develops into 

 a procercoid (Fig. 11.15). The body elongates and the hooks become 

 located in a posterior tail. Anteriorly a rostrum with very large apical 

 glands develops. When the arthropod is eaten by the appropriate second 

 host (fish, or other vertebrate), the procercoid sheds its tail, bores into 

 the tissue of the new host, and develops into a cercoid, which varies in 

 appearance in different tapeworms, but in general has a scolex and 

 somewhat resembles a miniature tapeworm without proglottids. In 

 tapeworms with a two-host cycle the oncosphere develops directly into 

 the cercoid stage. 



\Vhen the host with its cercoid larva is eaten by the appropriate 

 final host (usually a carnivorous fish, amphibian or mammal), the 

 larva attaches to the intestinal wall by the scolex and matures into a 

 tapeworm. Thus, the tapeworm cycle depends at each transition upon 

 being eaten by the next host. In some sj^ecies the cercoid stage is cap- 

 able of asexual multiplication, but in general each tapeworm e^'g pro- 

 duces a single adult worm. The number of eggs produced is tremendous. 

 For example Taenia saginata, a tapeworm that can infect man, sheds 

 8 or 9 proglottids daily, and each proglottid contains 80,000 eggs. The 

 infective larvae of this tapeworm occur in beef. 



Questions 



1. What kinds of organs are found anteriorly in flatworms? 



2. What influences the number of flame cells in the flatworm body? 



3. Describe the path of sperm from the testis to fertilization in Dugesia. 



4. What are the theories regarding the nature of polarity? 



5. Characterize the classes of the phylum Platyhelminthes. 



6. Describe the life cycle of a digenetic trematode. 



