46 ESSENTIALS OF ZOOLOGY 



are cosmopolitan in distribution, but D. lanceolatum is absent 

 from the British Isles. 



Distoma hepaticum has two suckers: one terminal and 

 anterior and containing the mouth ; the other, a short dis- 

 tance behind on the ventral surface, is large and projecting. 

 The outer surface is smooth, but the cuticle, which is secreted 







Mouth 

 Pharynx 



Sucker 

 Lateral nerve 



Opening of protonephridium. 



FIG. 19. Distoma hepaticum. Somewhat diagrammatic ventral view to show 

 the alimentary canal, the lateral nerve on the left, and the protonephridial 

 excretory system on the right. 



as a protective covering, is provided with numerous small 

 sharp projections directed posteriorly. Much of the structure 

 can be made out by examining the living worm in normal 

 salt solution and by staining and mounting. The mouth 

 leads into a short unpaired muscular pharynx, which opens 

 into the intestine, and the latter immediately divides into 

 two branches. These branches send out a large number of 

 secondary branching tubes, all ending blindly near the 

 margin of the body. The whole system is often seen clearly 

 mapped out when distended by bile from the liver of 

 the host. In D. lanceolatum there are no side branches to 



