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 gyna3cophore 

 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 Trcmatoda only multiply 

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

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

 genus Biporpa, two individuals (devoid of sexual organs) 

 conjugate, and the result is a double-bodied Diplozoun 

 parculoxitm, which ultimately develops sexual organs. 



In the case of Gyrodactyhis, a kind of internal gemmation 

 occurs. 



The Ccstoidca 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 Carijo- 

 phi/Uccus, have only a single set of hermaphrodite organs ; 

 and Ligida is an unsegmeuted 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 efferentia open into the common duct — the vas deferens ; 

 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. 



Tne female organ consists of the following parts : An 

 ovary leading into a single oviduct, which has an enlarged 

 portion or pouch, 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. 



