PHYLUM PLATYHELMINTHES 



167 



Xc/Ar 



Ov/Wucf 



IJ-f-erus 



5heJ/ 



row of hooks (Fig. 79). The proglottids represent individuals, and the 

 whole forms a colony, of which the section forming the scolex and neck 

 is the parent and in which new individuals are constantly being produced 

 by transverse constriction at the end of the neck. The proglottids are 

 carried farther and farther away from the point of origin as younger 

 individuals are produced and gradually become mature, with fully 

 developed sex organs. After the egg 

 cells are fertilized they develop in the 

 uterus. All other organs except the 

 uterus disappear 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 body 

 of the animal containing the parasite 

 with the feces. In the relation of the 

 members of a tapeworm colony to each 

 other there is a perfect correspondence 

 to the relation of the individuals in a 

 strobila. The scolex and neck corre- 

 spond to the scyphystoma and the 

 proglottids to the gradually develop- 

 ing ephyrae, both being cast off when 

 mature. New proglottids are devel- 

 oped where the neck ends, in the same ^'y H. w. Manter.) The egg cells are 



... , 1 1 1 1 a> formed in the ovary and passed into the 



fashion as new ephyrae are budded ott oviduct, where they are fertilized by 



from the SCVphvstoma. sperm cells from the seminal receptacle 



The effects of parasitism are carried 

 much farther than in the flukes. 

 Hooks may be added to the suckers 



as 



Fig. 77. — Diagrammatic sketch 

 showing relationships of female organs 

 in Clonorchis sinensis. (From a sketch 



and provided with yolk and shell-form- 

 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 

 organs of attachment, and the in the seminal receptacle have come 

 , .,, « ,, . ,. from another animal. In this case it is 



nervous system is still further simpli- 

 fied; but still more striking is the 



complete absence of the alimentary 



known that in copulation sperm cells 

 are introduced through Laurer's canal, 

 which opens to the outside. In some 

 other flukes, however, there is no ex- 

 Canal, the digested food in the intestine ternal opening to this canal and it seems 

 , . . J 1 j^ i_ • to be a vestigial organ. 



containing the tapeworm 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, which is scolex and neck. In contrast to this 

 form are others which may possess hundreds of proglottids and which 

 may reach a length of many feet. 



195. Metabolism. — The steps in metabolism in a free-living flatworm, 

 as illustrated in a planarian, have been seen to be not greatly different 

 in general character from those in the coelenterates. It should be noted 



