172 METAZOAN PHYLA 



4. If the egg falls into water which is at a temperature of about 24°C. 

 (75°F.), the embryo continues to develop and, after two or three weeks, 

 hatches, producing a ciliated larva known as a miracidium. This larva 

 resembles a free-living planarian in that it possesses a soft ciliated skin 

 and a double eyespot on the dorsal surface anteriorly. It also has a pair 

 of flame cells, a ganglion beneath the eyespots, and a pointed rostrum at 

 the anterior end. It swims about in the water but dies if it does not find 

 a snail of the right species within eight hours. 



5. Having found such a snail the miracidium enters its pulmonary 

 chamber and remains there. After about two weeks the parasite attaches 

 itself to the wall of this chamber by its rostrum, burrows into the tissues, 

 reaches the liver, and changes into a sporocyst. 



6. Within the sporocyst, from germ cells which, without fertilization, 

 develop and pass through the blastula and gastrula stages, there are 

 produced a number of rediae which escape from the sporocyst. Each 

 redia is provided with a mouth, an oral sucker, and a simple enteron. 



7. Within each redia, daughter rediae are produced from unfertilized 

 germ cells. There may be several generations of these. 



8. Finally a generation of rediae gives rise to a different type of larva 

 known as a cercaria, which is quite different in form from a redia. The 

 cercaria possesses a body and a tail, a ventral sucker in addition to the 

 oral sucker, and a branched enteron. 



9. The cercaria leaves the snail and after swimming about in the 

 water for a short time encysts on a bit of vegetation. At the time of 

 encystment the tail is lost, and within the cyst the structure gradually 

 comes to resemble that of the fluke, 



10. If this cyst is eaten by a sheep, the immature fluke is freed from 

 the cyst in the alimentary canal and by means of the bile ducts passes to 

 the liver, where it becomes mature. 



This life history is considered as representing one generation, the 

 different stages being larval forms. The sheep liver fluke, therefore, 

 exhibits a complicated metamorphosis. The reproduction which occurs 

 in the sporocyst and redia stages is interpreted as pedogenesis. 



There are trematodes which develop directly from the egg, but the 

 more usual life history includes several stages, and very frequently the 

 parasite has two, and sometimes even three, hosts, 



201. Life History of a Tapeworm. — The hfe history of a cestode 

 (Fig. 81) also involves two hosts, the final host becoming infected with 

 the parasite through eating the intermediate host. A typical life history 

 is that of the beef tapeworm. Taenia saginata (Goeze), the adult of which 

 is found in the human intestine, and which passes through the following 

 steps in its life history: 



1. The egg cell is produced in the ovary, passed into the oviduct, 

 fertilized, supphed with yolk, inclosed in a shell, and carried to the uterus. 



