Henn: South American P(eciliid Fishes. 97 



are limited to the Americas, it is the anal fin which serves as the intro- 

 mittent organ, it seems that in males of this species the ventral fins 

 have become modified into a large muscular appendage, or intromittent 

 organ. This might indicate that viviparity in this subfamily may be 

 of independent origin. 



Postnatal Development in the Subfamily Fceciliin^.. 



In most of the viviparous forms the sexes at birth are indistinguish- 

 able. The anal fin of the male occupies the same position as that of 

 the female, its shape is the same, and the individual rays are clearly 

 apparent. The location is usually below the posterior part of the 

 dorsal fin. As development proceeds, the third, fourth, and fifth rays 

 become lengthened, although they still remain separate and distinct 

 for some time. Eventually these attain their full length and appear 

 to be fused. While distinct, they are closely apposed and form a stiff- 

 ened rod or lever. The length varies with the difl^erent genera; usually 

 it is about one-third of the whole length of the fish. The tip of this 

 intromittent organ is provided with hooks and spines, the arrangement 

 of which difi"ers in the various genera. 



While these modifications have been taking place, the whole fin 

 has been gradually moving forward, so that, when development is 

 complete, the position of the fin has changed from abdominal ta 

 thoracic. The external openings of the genital tract, the ureter and 

 the intestine, which lie just before the anal, have also moved with it^ 

 This migration of the vent is evidently similar to that which takes- 

 place in the blind-fish {Amblyopsis spelams), where the opening of the 

 oviduct along with that of the intestine and the ureter move forward,, 

 so that the eggs may pass into the gill-chamber, where they are incu- 

 bated. The ventral fins also travel forward, and the final position of 

 these and of the anal is close up under the pectorals. 



A support, which is necessary for the mass of muscles involved in 

 the complicated movements of the anal fin during copulation, is pro- 

 vided in the males through a modification of the posterior praecaudal 

 vertebrae. A process or stay extends forward from each of the arches 

 uniting the parapophyses of the last few praecaudal or rib-bearing 

 vertebrae. In the vertebrae thus modified the ribs are absent, but their 

 places are taken by short processes which project backward in the 

 median line. The number of vertebrae bearing these stays varies 

 greatly. In Pmcilia vivipara (Fig. i) there are only two; in some forms 



