262 



PRACTICAL ZOOLOGY 



by the contraction of the pupil. The retina of the eye is stimu- 

 lated by the rays of light which pass through the pupil, and the 

 impulses which are carried through the optic nerve to the brain 

 give rise to sensations of sight. 



A 



Fig. 149. 



Urinogenital organs of the frog. 



A, male. 1, fat body ; 2, mesentery ; 3, efferent ducts of testis ; 4, ducts of 

 seminal vesicle; 5, seminal vesicle; 6, archincphric duct; 7, cloaca; 8, orifice 

 of ureter; 9, proctodeum; 10, allantoic bladder; 11, rectum; 12, kidney; 

 13, testis; 14, adrenal body. 



B, female. 1, oesophagus; 2, mouth of oviduct; 3, left lung; 4, fat body; 

 5, left ovary ; 6, archinephric duct ; 7, oviduct ; 8, allantoic bladder ; 9, cloaca ; 

 10, aperture of oviduct ; 11, aperture of archincphric duct ; 12, proctodeum ; 

 13, mesentery ; 14, kidney. (After Howes.) 



Reproduction. — With one or two possible exceptions all ver- 

 tebrates are bisexual. Individuals differ regarding their sex, 

 being either males or females, instead of being all of one sort like 

 the earthworm which is hermaphroditic and contains reproductive 

 organs of both sexes. The male frogs can be distinguished from 

 the females externally by the greater thickening of the inner 



