THE LIVER-FLUKES, TAPE-WORMS, ETC. 77 



This disease has been a serious scourge of sheep in Europe 

 for centuries, and for a long time has existed in Montana where 

 the loss is at least $10,000 every year. The number of the 

 parasites may be partially controlled by keeping only a few 

 dogs around the sheep or in regions where they are feeding, 

 and by keeping these dogs free from tape-worms. The dogs 

 should never be allowed to feed on the carcasses of the sheep 

 that have died from the disease, nor is it well to let them feed 

 on the heads of slaughtered sheep that may be infected. 



Classification of the Flat-worms. The Platyhelminthes are 

 divided into three classes, the Turbellaria, the Trematoda, 

 and the Cestoda. Another class, the Nemertea, is sometimes 

 included in this branch, but its relation is doubtful. Its 

 members are of no economic importance. 



The Turbellaria (L. turbellce, disturbance) are mostly non- 

 parasitic and have the epidermis covered with cilia. The 

 fresh-water planarians are examples. 



The Trematoda (Gr. trema, perforation; eidos, likeness) 

 are all either external or internal parasites. The life history, 

 especially of the internal parasites, is often very complicated, 

 as we have seen in our study of the liver-flukes. 



The Cestoda (Gr. kestos, a girdle, eidos, likeness) are all in- 

 ternal parasites in whose life history there occurs a tape- worm 

 stage in a vertebrate host and a bladder-worm stage in a 

 vertebrate or invertebrate host. The tape- worms of man and 

 of other animals are examples of this class. 



Parasites and Pearls. After calling attention to so much 

 harm that these lowly parasites may cause it is only fair that 

 a paragraph should be given to pointing out how a few of them 

 are of some service to man. For a long time it has been known 

 that the pearls that are found in many molluscs are secretions 

 formed about foreign particles that have found their way in- 

 side the shell. It is now known that the larvae of several 

 Trematode and Cestode worms are the objects around which 

 some of the finest pearls are formed. The Trematode larvas 

 are most common in mussels, while the Cestodes are found 

 very abundantly in the pearl oysters of Ceylon and other parts 

 of the world. The vertebrate hosts in these instances are 



