Phyla Platyhelminthes and Nemertina 419 



usual the mature proglottids pass out of the host with the feces. The 

 eggs contain ciHated embryos, the coracidia (Fig. 137,5), which are re- 

 leased in about ten days' time. Unless the embryos are consumed within 

 twenty-four hours by a small crustacean, a copepod, they will die. Once 

 the coracidium is eaten, however, it loses its cilia and bores through the 

 body cavity of the crustacean. Soon this embryo elongates and develops 

 hooks at the posterior end. It is now known as a procercoid (Fig. 



Fig. 137. — Structure and life history stages of some tapeworms. A-E, Diboth- 

 riocephalus latus: A, scolex, B, coracidium, C, young procercoid; D, mature pro- 

 cercoid; E, plerocercoid. F-G, Echinococcus granulosus: F, adult; G, section of 

 hydatid cyst. 



137,C and D). If the copepod is eaten by a fish, this procercoid grad- 

 ually gets to the liver of the fish and then works its way out to the flesh 

 of the animal where it becomes a wormlike plerocercoid larva (Fig. 

 137,£). If infected uncooked fish are eaten, the plerocercoid develops 

 into the adult in the human intestine. The pike and pickerel are the 

 favorite hosts in the northern portion of the United States. 



Echinococcus granulosus (Fig. 137,F and G) is a minute tape- 

 worm commonly found in the intestine of dogs, cats, wolves, and 



