PHYSIOLOGY OF THE INVERTEBRATA. 421 



curious mode of pairing occurs — for the male, not content to 

 unite with the female, retains the latter in a gynsecophore 

 or canal; but it may be stated that very little is known 

 concerning the reproduction and development of this Tre- 

 matode. 



So far we have seen that the Trematoda only multiply 

 sexually ; but some of these animals also multiply by conju- 

 gation and by a kind of gemmation. For instance, in. the 

 genus Diporpa, two individuals (devoid of sexual organs) 

 conjugate, and the result is a double-bodied Dvplozoon 

 paradoxum, which ultimately develops sexual organs. 



In the case of Gyrodactylus, a kind of internal gemmation 

 occurs. 



The Cestoidea are hermaphrodites in the mature condition, 

 but in the earlier stages of their growth they are devoid of 

 sexual organs (i.e., in the cystic form). Some, like Garyo- 

 phyllceus, have only a single set of hermaphrodite organs ; 

 and Ligula is an unsegmented form with many sets of these 

 organs. The Tape-worms are segmented animals, and in 

 each proglottis or segment there are male and female organs. 

 The male organ consists of innumerable pear-shaped vesicles 

 or testes, scattered in the parenchyma of the body. The 

 vasa efEerentia open into the common duct — the vas deferent ; 

 the latter lies in the cirrus sheath. By the contraction of the 

 cirrus sheath, the vas deferens (" cirrus ") can be forced 

 through the vagina.* In this case the vas deferens acts as a 

 copulatory organ or penis. 



Tue female organ consists of the following parts : An 

 ovary leading into a single oviduct, which has an enlarged 

 portion or poiich, termed the receptaculum seminis. Branch- 

 ing out from the oviduct are the vitellaria or yolk glands, 

 whose efferent tubes ultimately coalesce with the oviduct 

 forming a common duct. At the point where the ducts of 

 the vitellaria unite with the oviduct, the shell gland is 



* The vagina and vas deferens open into a cloaca or genital vestibule, 

 which is situated on the lateral margin of the proglottis. 



