PHYLUM PLATYHELMINTHES 



169 



Xa/Ar 



Uterus 



She// 

 ^/crna/ 



196. Cestoda. — This class includes the tapeworms, which are common 

 in the alimentaiy canals of vertebrates. A typical tapeworm (Fig. 83^) 

 consists of a more or le.ss rounded head, or scolex, and a relatively slender 

 neck, which together form one section of the body, and a number of other 

 sections called proglottids. The scolex bears suckers and is projected at 

 its free end into a rostellum, which may have an encircling row of hooks 

 (Fig. 84). Proglottids are constantly 

 being produced by transverse constric- 

 tion at the end of the neck. The pro- 

 glottids are carried farther and farther 

 away from the point of origin as 

 younger ones are produced and gradu- 

 ally become mature, with fully devel- 

 oped sex organs. After the eggs are 

 fertilized they develop in the uterus. 

 All other organs except the uterus dis- 

 appear and finally the gravid proglottid 

 becomes practically a sac of eggs, each 

 egg containing an embryo. Ultimately 

 these ripe proglottids are cast off and 

 are passed out of the host in the feces. 

 In the relation of the proglottids to 

 each other, there is some correspond- , ^'«- 82 —Diagrammatic sketch 



' . ... showing relationships of female organs 



ence to the relation of the individuals in Clonorchis sinensis. (From a sketch 



in a coelenterate strobila (Fig. 69F). ^^ h w Manter.) The egg cells are 



^ " formed in the ovary and passed into the 



The scolex and neck correspond to the oviduct, where they are fertQized by 



scyphistoma and the proglottids to the ^p^/™ """^^^ ^i"°™i^'' T^'^'f receptacle 



•' ^ . and provided with yolk and shell-form- 



gradually developing ephyrae, both be- 

 ing cast off when mature. Some zoolo- 

 gists, therefore, have interpreted a 

 tapeworm as representing a colony in 

 which the proglottids are individual ani- 

 mals. There is, however, considerable ^Z introduced through Laurer's canal, 

 ' which opens to the outside. In some 



unity m the organization of some SyS- other flukes, however, there is no ex- 



tems common to the entire tapeworm, t^^^i opening to this canal and it seems 



. . . to be a vestigial organ. 



The effects of parasitism are carried 

 much further in tapeworms than in the flukes. Hooks may be added to 

 the suckers as organs of attachment, and the nervous system is still fur- 

 ther simplified; but still more striking is the complete absence of the 

 alimentary canal, the digested food in the intestine containing the tape- 

 worm being absorbed through the body wall of the parasite. 



The simplest cestode lives in the body cavity of an annelid worm and 

 has only one section. In contrast to this form are others which may 



ing material from the yolk gland. They 

 then go on past the shell gland, the 

 secretion of which hardens the shells, 

 and into the uterus. The sperm cells 

 in the seminal receptacle have come 

 from another animal. In this case it is 

 known that in copulation sperm cells 



