HERMAPHRODITISM. 67 



unisexual. Sometimes a stem is hermaphrodite, but the individuals 

 unisexual (certain sertularians). Among jelly fishes the genus Cfirysa- 

 ora is known to be hermaphrodite. 



3. " Worms." — The condition of the sexual organs varies 

 enormously among the diverse types lumped together under the title 

 of "worms" or "Vermes." In the lowly turbellarians, all the 

 genera are hermaphrodite except two; but, as in many other cases, the 

 organs do not reach maturity at the same time, the male preceding. 

 In the related trematodes or flukes, hermaphroditism again obtains, 

 with one exception, or perhaps two. The certain exception is the 

 curious parasite Bilharzia, where the male carries the female about 

 with him in a "gynaecophoric canal," formed of folds of skin. In the 



Fig. 16. — Bilharzia, a parasitic trematode, in which the male carries 

 the female in a special fold of skin called the "gynaecophoric canal." 

 After Leuckart. 



adjacent class of cestodes or tapeworms, all the members are hermaph- 

 rodite. These three classes are doubtless related; but it seems 

 plausible to connect the retention of hermaphroditism with the 

 degeneracy of parasitism, and also with the rich yet at the same time 

 stimulating nutrition, which may favor the retention of double 

 sexuality. The utility of the hermaphrodite state, if the eggs of these 

 animals are to be fertilized and the species maintained, can hardly be 

 doubted, but this does not explain the facts. It is important to 

 notice too, that self-fertilization — that is, union of the eggs and sperms 

 of the same organism — has been proved to occur in several 

 trematodes, and seems to be almost universal in cestodes. This may 

 be one of the conditions of the degeneracy of these parasites, for 

 frequent as hermaphroditism is among plants and animals, self- 

 fertilization is extremely rare. 



Hermaphroditism is rare among the free-living nemerteans, but 

 constant in the semiparasitic leeches. The only exception to separate- 

 ness of the sexes among threadworms or nematodes is the very 

 curious case of the genus Angiostomum. Here, in an organism which 

 is anatomically a female, the reproductive organ starts with produc- 

 ing spermatozoa, which fertilize the subsequent ova. The animal is 



