70 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



from them by the absence of rings around the body. These 

 planarians are less than half an inch long, very thin and 

 rather broad. On the upper surface near the front is a pair 

 of pigmented spots which are probably sensitive to light and 

 are called the eyes. The mouth is on the under surface a 

 little behind the middle of the body. The alimentary canal 

 is composed of three main branches, each with numerous 

 small side branches. One main branch runs forward from 

 the mouth, and the other two run backward, one on each side 

 of the body. There is no anal opening, and the alimentary 

 canal thus forms a system of fine branches closed at the 

 tips, and extending all through the body. The nervous 

 system is composed of a ganglion or brain in the front end of 



FIG. 18. A fresh-water planarian, Planaria sp. (Eight times natural 

 size; from a living specimen.) 



the body from which two main branches extend back through- 

 out its whole length. From these main longitudinal branches 

 arise many fine lateral branches. 



Many beautiful and interesting members of the class 

 Turbellaria the class to which the planarians belong are 

 marine and a few are found in moist earth. 



Liver-flukes. The liver-flukes, Fasciola hepatica, live as 

 parasites in the liver of sheep and cattle, especially the former. 

 In Europe they sometimes kill hundreds of thousands of sheep 

 annually, and they have more recently become of some import- 

 ance in the United States, the Pacific and the Gulf Coast 

 regions suffering most. They are interesting not only on 

 account of their economic importance but because they furnish 

 a good example of a Metazoan parasite that requires two 

 different kinds of hosts in which to complete the different stages 



