THE FLATWORMS— PLATYHELM1NTHES 147 



they frequently do because many of the houses are built over or near 

 the water and the rivers and lakes offer a convenient means of dis- 

 posal of sewage. If snails of the right species eat these eggs, they 

 hatch into tiny larvae (miracidia) which go through a series of asexual 

 reproductive phases within the snail's body (sporocysts and redia). 

 Eventually, a large number of larvae (cercariae) possessing tails for 

 swimming are produced by this method. These break out of the 

 snail and swim around in the water aimlessly until they come in con- 

 tact with a fish; they then burrow through the skin of the fish and 

 encyst in its muscles (encysted cercariae). Many of the people in 

 these oriental countries practically never cook fish. It may be sliced 

 and placed on top of steaming rice to warm it up a little or perhaps 

 dipped in a hot sauce that tends to disguise the "fishy" raw taste, but the 

 encysted larval flukes are not destroyed by such treatment. When they 

 reach the stomach the cyst wall is digested off and, as they pass into 

 the small intestine, the little wiggling larvae find the bile duct and 

 swim up it to the liver. Here they grow into adulthood along with 

 their relatives that have survived this precarious journey from one 

 person to another. In some regions of the orient this fluke infects 

 more than 75 per cent of the inhabitants. 



The human liver fluke is not able to spread in the United States. 

 There is a sheep liver fluke (Fasciola hepatica), however, that is rather 

 abundant and often causes serious losses of sheep. The snail also acts 

 as an intermediate host for this parasite, but the larvae (cercariae) from 

 the snail do not go to fish for encystment. Rather they crawl up a blade 

 of grass or other succulent plant which grows up out of the water. Here 

 they may be eaten by sheep and the cycle is completed. 



The flukes represent another case of alternation of generations such 

 as was first studied in obelia in the coelenterates. There is a sexual 

 generation alternating with asexual generations. 



Tapeworms — The Cestoda 



This class includes the tapeworms that are so very abundant in prac- 

 tically all mammals and many of the other vertebrates. Their bodies are 

 more specialized than the flukes, and they have very little similarity to 

 planaria externally. The body of an adult tapeworm consists of a head, 

 or scolex, which is attached to the intestinal wall of its host with the aid 

 of suckers and sometimes hooks, and a series of many segments, or pro- 

 glottids, which come off from the neck of the scolex and trail out into 

 the intestine, usually for several feet. It somewhat resembles a tape 

 measure, from which it derived its name. Because tapeworms are 



