116 INVERTEBRATE ZOOLOGY 



A cercaria (D) is a minute fluke which in addition to the rudi- 

 mentary organs of the adult contains also a strongly developed 

 posterior tail for locomotion. The cercariae leave the body 

 of the snail and for a time are free-swimming creatures. Finally 

 the cercaria crawls onto vegetation where after losing its tail it 

 becomes surrounded by a calcareous cyst wall (E). In this 

 condition, it remains inactive until the plant bearing the cyst is 

 taken into the stomach of a sheep or other suitable animal. 

 Under the digestive action, the young fluke is liberated from the 

 cyst and occurs free in the digestive tract. As it passes into 

 the intestine, its chance of ever reaching maturity rests 

 upon the discovery of the opening of the bile duct and migration 

 through it into the liver. The successful individual becomes 

 established in the bile passages where after a few weeks of growth 

 it has reached adult size and begins to produce eggs, thus closing 

 the cycle. 



In the foregoing instance the cercaria enters its vertebrate 

 host in a purely passive manner. Similarly some of the most 

 dangerous human flukes, especially in the Orient, are taken into 

 the digestive tract in an encysted state. In contrast, the blood 

 flukes belonging to the genus Schistosoma have active cercariae 

 which gain entrance to the human body by penetrating the skin. 



In so far as is known, a mollusc is essential for the development 

 of all digenetic trematodes. Frequently, other hosts are added, 

 either as essential links in the life cycle or as faculative adapta- 

 tions, until the complete developmental cycle involves a number 

 of different species which act as host to a single parasite. 



Class Cestoda 



Like the trematodes, all cestodes are parasitic but through 

 fundamental structures show marked relationships with the less 

 highly specialized representatives of this phylum. The only 

 sure criterion for the separation of Cestoda from other parasitic 

 flatworms is their entire lack of a digestive system. While the 

 typical cestode is made up of a chain of segments or proglottids 

 and a scolex for attachment to the host, there are some, the 

 monozoic cestodes or Cestodaria, in which the body is composed 

 of but a single unit. Though these latter are usually not more 

 than a few millimeters in length, the segmented forms frequently 

 attain a length of several meters and may be divided into several 

 thousand proglottids. 



