The Sexual Conditions of Myzostoma glabrum (F. S. Leuckart). 321 



than it perhaps deserved, I bave met with nothing at ali seriously 

 railitating agaìnst my originai view. 



There are only tvvo possible alternatives as to the herma- 

 phroditism of the genus Myzostotna^ that the hermaphroditism is 

 primary, or that it has been derived from a dioecious condition. 

 If it be primary, the existence of small complemental males, among 

 other tbings, cannot be accounted for in view of what is really 

 knowu regarding them in M. cjlahrum and M. puhinar. But, if it 

 has been derived from a dioecious condition, these small males 

 admit of easy explanation. 



If it be primary, we meet with difficulties in the »peculiar< 

 conditions presented by M. puhinar. In fact, it then becomes dif- 

 ficult to understand why there should be any dioecious species at 

 ali in the group; for, although many zoologists bave recognised in 

 the parasitic mode of life a tendency to the production or evolution 

 of hermaphroditism, no-one, with the possible exceptiou of Wheeler, 

 has yet seen in it any leaning towards the evolution of separate sexes. 



Assuming the males of a dioecious group to become small and 

 »complemental« with the Initiation of hermaphroditism in the females, 

 there are iu the long run only two courses open to the former: 

 when the hermaphroditism of the originai females becomes perfect, 

 they may disappear, as useless organisms, or they may themselves 

 become hermaphrodite. 



Practically it is the latter which happens; for, although we 

 have not yet probed to the bottom what determines the sex of a 

 young form, there can, I think, be little question, that the results of 

 Amphimixis will come into play, and that, with the diminution in 

 the number of eggs fertilised by the true males, more especially, 

 with the increasing irregularity with which eggs of consecutive 

 generations are fertilised by the true males, i. e., with the great 

 preponderance of the spermatozoa of hermaphrodites over those of 

 the males, there will be fewer and fewer males produced, and, 

 finally, they will disappear. 



In parthenogenesis , as Weismann has shown, the appearance 

 of males tends to become cyclical, but enormous periods of time will 

 be required before the cycle becomes infinite, and the males disappear. 

 In hermaphroditism there is oue thing, which tends to preserve the 

 males, and this is the dose association, such as occurs in M. glabrum 

 to some extent, and, more particularly, in M. pulvhiar, of male and 

 hermaphrodite, or of male and female. In this way it is practically 



Mittheilungen a. d. Zool. Station zn Neapel. Bd. 13. 21 



