THE PHYLUM PLATYHELMINTHES 



215 



pair of eyes. It apparently has no digestive tract at all although it has a 

 typical set oi flame cells. The anterior rostrum lacks cilia and is equipped 

 with an apical gland that secretes corrosive juices for penetrating the 

 tissues of the host. The body is filled with reproductive tissue. The 

 miracidium darts about rapidly in the \vater, and if it fails to find the 

 proper species of snail in a few hours it will die. 



As the miracidium penetrates the snail it sheds its ciliated epi- 

 dermis and rounds up as a sporocyst (Fig- 11.13) covered with a thin 

 cuticle. All miracidial structures disappear except some subepithelial 

 muscle fibers and the flame cells, while the reproductive tissue develops 

 into a variable number of embryos. Nutrients are absorbed from the 

 host directly through the cuticular wall. 



Each embryo develops into the next stage, usually a redia (Fig. 

 11.13). This escapes from the sporocyst and begins to feed upon the 

 tissues of the host. The redia has an anterior mouth, a muscular pharynx 

 by which host tissue is sucked up, and a short, saclike intestine. The 

 body wall is made up of a cuticle, muscle and mesenchyme. A brain 

 with nerve cords and a flame cell system are also present. The rest of 

 the body is fdled, as in the miracidium, with reproductive tissue. Again, 

 this tissue develops into a number of embryos. 



Figure 11.13. Life cycle of a digenetic fluke. The stages shown here belong to vari- 

 ous species. The arrows indicate whether one stage becomes the next or whether it pro- 

 duces the next by reproduction. (After Hyman.) 



