PHYLUM PLATHELMINTHES 



119 



on the ventral surface. When it is marginal there is no necessary 

 regularity in its disposition, for in adjacent proglottids it may be 

 either on the same or on opposite margins. In cestodes which 



Genih/pore. 

 . Cirrus. 



.Jesfis 



Uterus,-' 



Ovar^ --'.' 



_ . .. Shell gland 



'~ ■Seminal rTecepfade - 



.'•^Vilellan'a ■ 



Laurers canal 



Fig. 66. — Diagrams showing the fundamental plan of organization of sexual 

 organs in ^-1, trematodes; B, cestodes. (Grig.). 



have two sets of genital organs, the pores occur on both lateral 

 margins of each proglottid. 



The arrangement of the female organs differs greatly in the 

 different genera. The vagina opens through the common genital 



/ transverse 

 excretory duct 



vitelline duct 



cirrus sac 

 ^genital atrium 



vagina 



longitudinal ' uterus 



testicular follicles vas deferens 



circular muscles 



muscles 



Fig. 67. — Stereogram showing arrangement of organs in a proglottid of a fish 

 tapeworm (Thysanocephalum). (After Causey). 



pore and in addition in some instances the uterus has an inde- 

 pendent orifice to the exterior. A typical arrangement (Fig. 

 67) is as follows: The vagina, as it passes through the proglottid, 



