Phyla Platyhelminthes and Nemertina 411 



the redia. There are three to eight redia within each sporocyst. These 

 rupture the sporocyst and migrate to the liver of the snail. Each redia 

 is also a saclike structure, but in addition has a mouth and short gut. 

 For a short time, each redia is capable of producing parthenogenetically 

 additional redia. Each redia finally produces several larvae of another 

 type, the cercariae. In some instances, the original redia produces cer- 

 cariae directly; at other times, the mother redia produces daughter 

 rediae which produce the cercariae. As many as 300 larvae may be pro- 

 duced in this way from a single tgg. 



The individual cercaria bores out of the snail and becomes free 

 swimming. The cercaria of Fasciola hepatia is a disc-shaped animal with 

 a long tail ; it has an oral and a ventral sucker. After a few hours, the 

 cercaria attaches to a piece of grass, loses its tail, and produces a tough 

 outer covering. This cyst is now known as the metacercaria, and is able 

 to remain viable for several months. If a sheep happens to eat grass on 

 which metacercariae are encysted, the metacercariae pass into the diges- 

 tive tract where the cyst wall is digested off by the digestive enzymes. 

 The worm now released burrows through the intestinal wall into the 

 coelom, and migrates to the liver. Here it becomes an adult, and the 

 life cycle may be repeated. 



This fluke is most commonly found in the sheep, but may occur in 

 cattle and has been reported in man. 



On the whole, the anatomy of the adult sheep liver fluke is very 

 difficult to study or to describe as the development of the coiled uterus 

 and the branched intestine obscures most of the details. 



The Anatomy of Opisthorchis Sinensis. — Fortunately there are 

 several forms which have a less enlarged uterus and anatomical studies 

 can be made on them. Such a fluke is pisthorchis sinensis (formerly 

 Clonorchis sinensis), the adult of which is found in carnivores in the 

 Orient. Frequently infections are also found in human beings who eat 

 raw fish. 



This worm attaches by means of two external suckers, an oral 

 at the anterior end and a ventral sucker. The digestive tract consists 

 of a mouth in the center of the oral sucker, a narrow pre pharynx, a 

 muscular pharynx, a short esophagus, and two intestinal cecae. These 

 intestinal cecae end blindly in the posterior region of the body. The 

 nervous system consists only of a small ganglion at the anterior end 

 of the body and a few longitudinal nerves. There are no sense organs, 

 but these would be relatively useless in an animal living within the bile 



