208 



MANUAL OF ELEMENTAL?} ZOOLOGY 



three kinds of individual involved in the cycle. The life- 

 history of the liver fluke is shown by a diagram in Fig. 126. 



I - qenerahon 

 2 - d qenerarion 

 3^ generation 



:aa > Miracidium ^SporocysT ■ 



\ 

 Red! a \^^ Sporocysr 



Cercaria — ?-Adulr Redia 



Fig. 126. — A diagram of the life-history of the liver fluke. 



Platy- 

 helmlnthes. 



The liver fluke belongs to a group of animals known as 

 Platyhelminth.es. These are flat, bilaterally symmetrical 



worms, without anus, 

 blood vessels, or 

 body cavity in the 

 adult, with an excretory system 

 formed of branched tubes end- 

 ing in flame cells, and with a 

 complicated system of repro- 

 ductive organs. Some of them, 

 known as Turbellaria, lead a 

 free life, and are ciliated all 

 over. Others, like the liver 

 fluke, are parasitic and covered 

 with a cuticle, and possess 

 suckers. These are known as 

 Trematoda. As another ex- 

 ample of this group, we may 

 notice Schistostomum (or Bil- 

 harzid), a trematode which lives 

 in the veins of man and is re- 

 markable for the fact that the 

 female is carried in a groove 

 of the ventral surface of the 

 male. Eggs are laid in the 

 walls of the intestine and 

 bladder, causing inflammation, 

 and thus reach the exterior. The intermediate host is a 

 snail, from which the cercarise pass into water, Infection 



Fig. 127. — Schistostomum 

 haematobium . — From 

 Sedgwick. 



<$ male ; £ , female ; 6", sucker. 



