174 METAZOAN PHYLA 



u1;ems of the fluke, where development begins. The egg, containing an 

 embryo, is laid through the genital pore of the fluke and passes out of the 

 body of the sheep with the feces, by way of the bile ducts and intestine. 



Under favorable conditions of moisture and temperature the embryo 

 within the egg continues its development into a larval form known as a 

 miracidium. This usually requires two or three weeks. The miracidium 

 hatches from the egg only in water. It is a microscopic, but multicellular, 

 ciliated creature with a pointed rostrum at the anterior end and a double 

 eyespot on the dorsal surface anteriorly. The miracidium swims about 

 in the water but cannot feed and dies within about eight hours unless it 

 locates a snail of the right species. Probably most miracidia weaken and 

 perish for want of the lifesaving snail. 



Having found a snail, the miracidium enters its pulmonary chamber 

 and later burrows into the soft tissues of the snail to the liver. Within 

 the snail, the miracidium sheds its coat of ciha and becomes transformed 

 into a sacHke form known as the sporocyst. The sporocyst is the second 

 larval form in the hfe cycle. Within it are groups of cells, usually con- 

 sidered as germ cells, which develop, without fertiUzation, into larvae 

 known as rediae (singular, redia). The sporocyst, therefore, carries on 

 reproduction. The redia is also sachke but, unhke the sporocyst, pos- 

 sesses a mouth and a simple enteron. Rediae escape from the sporocyst 

 and develop in the hver of the snail. There may be several generations of 

 rediae, the daughter rediae arising from unfertilized germ cells. 



Finally, rediae give rise to a very different type of larva known as the 

 cercaria also developing from what seem to be unfertihzed germ cells. 

 The cercaria possesses a body and a tail, the body having an oral and a 

 ventral sucker and a forked enteron hke the adult. It is the fourth type of 

 larva in the life cycle. It burrows its way out of the snail into the water 

 where it swims about actively by lashing its tail back and forth. Because 

 of the reproduction by the sporocyst and rediae, a snail infected by a single 

 miracidium may later shed large numbers of cercariae. 



The cercaria swims about for a short time only, then settles down on a 

 bit of vegetation, sheds its tail, and secretes a cyst wall entirely enclosing 

 the body portion. If this cyst is eaten by a sheep, the immature fluke is 

 freed from the cyst in the alimentary canal. According to recent studies, 

 it penetrates through the wall of the digestive tract into the body cavity 

 and thence into the hver, finally arriving in the bile ducts. 



This curious and rather complicated life cycle is followed with some 

 modifications by almost all trematodes. While the life of the adult may 

 be monotonous, the youthful career is dangerous and eventful. Only a 

 few eggs are fortunate enough to hatch, only a few of the miracidia 

 succeed in finding a home in a snail, and most of the encysted cercariae are 

 doomed never to awaken for lack of the magic touch of the digestive 

 juices of the final host. 



