454 



ESSENTIALS OF ZOOLOGY 



probably more correct to consider a tapeworm as a linear colony, in 

 which the segments are really individuals in various stages of maturity. 

 Taenia solium, the pork tapeworm of man, may be taken as an 

 example to illustrate the structure and life history of a cestode. The 

 adult tapeworm consists of a scolex or head provided with four mus- 

 cular suckers and a snoutlike 7'ostellum surrounded by a row of 

 chitinous hooks, which serve as means of attachment to the wall of 

 the human intestine; a narrow unsegmented neck behind the scolex, 

 and then a series of several hundred proglottids (the segments) be- 

 coming progressively larger as they get farther from the scolex. The 



AC D 



Fig. 194. — Development of tapeworm. A, six-hooked embryo ready to become 

 embedded in muscle ; B, cysticerus, or bladder worm as encysLCd ; C, section tlirou^^h 

 developing scolex in cysticercus ; D, later stase ; E, scolex everting as it protrudes 

 from bladder ; F, extension of scolex from bladder ; G, later stage ; H, formation of 

 proglottids. (From Parker and Haswell, Textbook of Zoology^ The Macmillan 

 Company, after Jijima and Hatschek.) 



whole chain of proglottids is called the strohilus. New proglottids 

 are constantly budded off from the neck; consequently, the youngest 

 proglottid is the first one back of the neck and the oldest one is 

 the one at the end of the strobilus farthest from the scolex. The 

 youngest proglottids contain no recognizable structures, except the 

 paired longitudinal nerve cords and longitudinal excretory vessels 



