CESTODA. 249 



placed at the side, the female on the surface of the segment. In 

 other cases there are two sets of generative organs in each segment, 

 opening on the right and left margins (Dipylidium). In the Bothrio- 

 cephalidae and other forms (possibly in all Cestodes except the 

 Taeniadae) the uterus has a special opening of its own to the exterior 

 in addition to the vagina, which opens close to the vas def erens 

 (Fig. 216). This opening is comparable to the uterine opening in 

 Trematoda, differing from the latter in being remote from the 

 male opening. The vagina of Cestodes, which is not used for the 

 exit of eggs, even when there is no uterine opening (see above, 

 p. 244), must be compared to the canal of Laurer in the Trematoda. 

 In forms with a special uterine opening, eggs are deposited through 

 it while the proglottis is part of the chain in the intestine. When 

 there is no uterine opening, the eggs are only set free by the 

 rupture of the proglottis after it has broken away from the chain 

 and reached the exterior. 



As the segments increase in size and become further removed from 

 the head, the contained generative organs gradually reach maturity 

 in such a way that the male generative organs arrive at maturity 

 rather earlier than the female. As soon as the male elements are 

 mature, copulation is said to take place, and the receptaculum seminis 

 is filled with sperm, and then only do the female generative organs 

 reach maturity. 



The method of sperm -transference is not fully understood. The penis is 

 sometimes found projecting from the genital opening (Fig. 207), and Leuckart 

 states that he has seen it inserted into the vagina of the same proglottis, thus 

 being in a position to effect self-fertilization. In other cases it has been found 

 (Pagenstecher) inserted into the vagina of another proglottis of the same chain. 

 There is also the possibility of copulation between the proglottides of different 

 chains in the same host, though this has not been observed ; and it may be that 

 in some cases the penis is used, like that of the Turbellaria, for hypodermic 

 injection of spermatozoa. Finally we must admit the possibility of the sperm 

 passing out into the fluids of the intestine in which the body of the tape-worm 

 is bathed, and of the spermatozoa so set free migrating in sufficient numbers 

 into the vagina of other proglottides. 



The ova are fertilized and pass into the uterus, which then assumes 

 its characteristic form and size. As the uterus becomes distended, 

 the testes and then the ovaries and vitellaria are more or less com- 

 pletely absorbed (Fig. 202). The posterior proglottides, viz., those 

 which are ready for separation, have alone undergone full develop- 

 ment, and the eggs in their uterus often contain completely developed 

 embryos. Accordingly we can recognize in a continuous series of the 



