244 ELEMENTS OF COMPARATIVE ZOOLOGY. 



having sucking discs (from one to many) developed upon the 

 body. Some of them become serious pests. One form, the 

 liver-fluke, produces the disease known as " liver-rot" in 

 sheep. Other forms occur in man, especially in the tropics, 

 being introduced in drinking-water. They cause serious 

 sickness. 



ORDER III. CESTODES (Tapeworms). 



The Cestodes are all parasitic in other animals. They 

 differ from the Trematodes in the complete absence of 

 mouth and digestive tract, since they absorb their nourish- 

 ment through the skin. Usually they have ribbon-like 

 bodies, and hence are commonly known as tapeworms. At 

 the anterior end are the means of attach- 

 ment (hooks or suckers) by which the 

 animal attaches itself to the lining of 

 the intestine of its host, while usually 

 the body becomes broken up into a se- 

 ries of joints or proglottids. There is 

 continually a formation of new proglot- 

 tids near the head, while the older pro- 

 glottids, loaded with eggs, drop off and 

 are carried out with the waste of the 

 digestive tract. These tapeworms ob- 

 tain entrance into the body in the food, 

 man usually receiving his from raw or 

 partially cooked beef or pork, and more 

 rarely from fish. The proglottids and 



FIG. 115. T a p e w o r m 



(Tmnia) with progiot- eggs, passing from the body, may fall 



tids from different re- 

 gions of the body, /i, where they may be eaten by cattle or 



head enlarged. . J . n ^ . J 



swine. Inside their bodies, they undergo 

 partial development in the muscles, and then when taken 

 into the human body they complete their development. 



