216 ^HE ANIMAL KINGDOM 



Each embryo within the redia may develop into another redia, or 

 into the next stage, a cercaria (Fig. 11.13), which escapes from the redia 

 through a birth pore. Each cercaria is a miniature fluke with a tail. At 

 the front end it has a penetration stylet equipped with an apical gland. 

 The cercaria leaves the snail and swims through the water by lashing 

 its tail, searching randomly for the next host, which varies considerably 

 (crayfish, clam, fish, etc.), according to the species of fluke. The cercaria 

 bores into the new host, sheds its tail, and becomes surrounded by a 

 cyst. 



Within the cyst the stylet and apical glands disappear, and the 

 other structures develop further toward the adult pattern. This stage, 

 the metacercaria (Fig. 11.13), must be eaten by the final host in order 

 to mature. Thus, the fluke does not feed upon this second host, in 

 which the cercaria becomes a metacercaria. The second host serves as 

 a means of gaining entry into the final host, which is usually some kind 

 of vertebrate carnivore (fish, frog, cat, man, etc.). In some species the 

 cercariae encyst and become metacercariae on aquatic vegetation, and 

 are thus able to parasitize an herbivore (sheep, cow, etc.) as the final 

 host. 



When the metacercaria is eaten by the appropriate final host, the 

 cyst wall dissolves in the latter's intestine, and the young fluke emerges. 

 It then migrates through the body to its final site (lungs, liver, etc.), 

 feeding and growing as it goes, and finally maturing in a few days to 

 several weeks. 



The details of reproduction in the sporocyst and redia have been 

 difficult to interpret, and hence are subjects of considerable controversy. 

 If the reproductive tissues of sporocyts and rediae produce eggs that 

 become rediae and cercariae, then the life cycle involves three genera- 

 tions of organisms. If, on the other hand, the reproductive tissues are 

 simply persistent embryonic tissue that divides to form many indi- 

 viduals, reproduction in the sporocyst and redia is similar to that of 

 the coelenterate polyp, and the entire cycle is a single generation with 

 asexual reproduction in larval stages. The distinction rests upon whether 

 or not meiosis occurs during larval reproduction, an issue that has not 

 yet been settled. 



80. Class Cestoda 



Tapeworms are endoparasitic flatworms without epidermis, mouth 

 or digestive tract. The front end of the body is a knoblike scolex, armed 

 with hooks or suckers by which the animal attaches to the host (Fig. 

 11.14). Behind the scolex is a narrow neck, followed by a long chain of 

 proglottids. Proglottids are produced by segmentation in the neck re- 

 gion, where rapid longitudinal growth takes place constantly. As each 

 proglottid ages, it is found farther and farther back along the length 

 of the worm. It widens and lengthens, and eventually becomes mature. 

 Each proglottid has a complete set of reproductive organs, similar to 

 those of the Turbellaria except that the genital opening is lateral. As 



