BREEDING BEHAVIOR OF SUCKERS AND MINNOWS. 7 



excellent field characters by which the sexes of the breeding 

 fish may be distinguished in their native waters. Less striking 

 differential characters are the greater length of the caudal and 

 lower fins of the male, the difference in length of his caudal lobes 

 and his possession of pearl organs. 



Effective pearl organs (vide introduction) occur on the male 

 in the following situations (Fig. i): (i) Large, sharp-pointed 





FIG. i. Lateral view of a part of a male and female of Catostomus commersomi 

 drawn to the same scale. The black dots represent pearl organs. The anal fin 

 and lower lobe of the caudal are longer in the male and bear large pearl organs . 

 All the scales of his sides bear small pearls, somewhat larger behind the caudal 

 The female has no pearl organs. 



organs are found on the anal fin and on the lower part of the 

 caudal. They may be visible to the unaided eye at a distance 

 of three or four yards. (2) The caudal margins of the scales 

 on the sides bear small hemispherical organs, which are effective 

 behind the dorsal. (3) The upper surfaces of the pectorals and 

 both surfaces of the pelvic fins bear small organs. (4) The rays 

 of the dorsal fin bear small organs. Pearl organs do not occur 

 in the female (Fig. i). 



