184 SEXUAL DIMORPHISM 



male is thus modified, but is not tubular. Nothing is stated 

 as to the use of the enlarged dorsal in the male. In young 

 males the dorsal fin is not longer than that of the female. 

 Similar remarks apply to Xiphophorus, in which also the 

 modified portion of the anal is not tubular. In each case, 

 the peculiarity of the dorsal or caudal in the male may be of 

 special use in producing the proper movements of the body 

 for the purpose of copulation. 



The non-tubular modification of the anal fin in males 

 occurs also in the sub-family Gambusiinse, as well as in 

 many other genera placed with Mollienisia and Xiphophorus 

 in the sub-family Poeciliinse. Observations on the copula- 

 tion of Gambusia are mentioned by Ryder in his paper on 

 the development of this form ; the male is stated to have his 

 head turned towards the tail of the female, and as the intro- 

 mittent organ slopes backwards, this position would seem 

 necessary ; it may be suspected that the ventral edges of the 

 pair are turned towards each other, otherwise it is difficult 

 to understand how the projection of the fin is inserted into 

 the female opening. In Gambusia and its allies, however, 

 the males have no very important peculiarities in addition 

 to the intromittent organ. In nearly all cases they are much 

 smaller and less numerous than the females. 



In Jenynsia and Anableps a tube continued from the 

 urinogenital ducts runs down the front of the elongated 

 portion of the anal fin, and thus forms a more specialised 

 intromittent organ. In both cases copulation evidently 

 takes place sideways, as there is a difference between the 

 two sides of the copulatory organs which enables specimens 

 to be distinguished as rights and lefts. Both rights and lefts 

 occur, however, in each sex ; the males are not all rights, and 

 females all lefts, or vice versa. 



In Anableps the structure of the anal, after its modifica- 

 tion, which is not developed until some time after birth, is 



