PHYLUM PLATYHELMINTHES. SIMPLE ORGAN-SYSTEM ANIMALS 



143 



Mai 



Testis 



Ventral 

 sucker 



Oral 

 sucker 



Adult schistosomes in blood vessels 

 of bladder and urinary tract 



Eggs are passed in urine 

 and hatch into miracidia 

 which enter snail 



Cercariae emerge from 

 snail and penetrate the 

 exposed skin of man 



\ 



Cercaria 



Free-swimming 

 miracidium 



Sporocyst stage in snail 



Figure 75. Life cycle of a human blood fluke (Schistosoma haematobium). The large male 

 is carrying the small female. This is an ancient human parasite, especially common in many 

 parts of Africa and lower Egypt. 



Why is the tapeworm not digested in the 

 human intestinal juices (enzymes)? It is be- 

 Heved that the tapeworm is protected from 

 the action of digestive enzymes by means of 

 an anti-enzyme mechanism. 



ORIGIN AND RELATIONS OF 

 THE PLATYHELMINTHES 



The flatworms, especially the Turbellaria, 

 resemble the coelenterates in certain re- 

 pects which indicate coelenteratelike ances- 

 tors. There is usually a single opening for 

 ingestion of food and egestion of waste ma- 



terial; a nerve net reminiscent of the coelen- 

 terates; and the parenchymal connective tis- 

 sue is similar to the cellular mesoglea in 

 higher coelenterates and ctenophorcs. The 

 chief differences between the classes of flat- 

 worms appear to be due to their free-living 

 or parasitic character. 



RELATIONS OF THE 

 PLATYHELMINTHES TO MAN 



The free-living flatworms are of very little 

 importance to man, but many of the trema- 

 todes and cestodes are dangerous parasites, 



