298 



A HISTORY OF FISHES 



be extruded, and are probably drawn into her body by some 

 sort of sucking action. 



All Cyprinodonts with complicated intromittent organs retain 

 the fertilised ova within their bodies, and the young are brought 

 forth alive, but there are a number of forms in which eggs are 

 extruded and fertilisation takes place externally. Here the 

 differences between the sexes are concerned merely with the 

 coloration or with the shape of the fins (Fig. io8b), but there is a 

 defmite pairing of male and female, usually an elaborate court- 

 ship, and intercourse takes place in the following manner. The 

 two fishes lie side by side, the heads looking in the same direction, 

 and the male clasps his partner by folding his dorsal and anal fins 



Fig. 1 08. SEXUAL DIFFERENCES IN CYPRINODONTS. 



A. Male and female of Gamhusia nicaraguensis, X i ; b. Male and female of 

 Nothobranchius arnoldi, X i . 



across her body, while the paired fins also may interlock. The 

 ova and sperms are then extruded simultaneously in such close 

 proximity that fertihsation of the vast majority of the eggs is 

 certain. It seems probable that the curious forms with a 

 priapium may take up a similar position during the sexual 

 act, but the behaviour of these fishes in the breeding season 

 has not yet been observed, and the actual function of the organ 

 remains a matter for speculation. 



The second type of differences between the sexes, the secondary 

 sexual characters, which have no connection with actual union 

 between male and female, may be present during the whole 

 adult life of the male, but are frequently developed only as the 

 spawning season approaches, and are discarded as soon as this 

 is over. The difference in the size of the two sexes is worthy 



