URINO-GENITAL SYSTEM OF THE FROG. 459 



walls. The cavity is lessened by the spongy nature of the 

 internal walls, which form numerous little chambers bearing 

 the fine branches of blood-vessels. 



In respiration, the mouth is kept shut, and air passes in 

 and out through the nostrils. A frog will die of asphyxia 

 if its mouth be artificially kept open for a considerable 

 time. When the floor of the mouth is lowered, and the 

 buccal cavity thus increased, air passes in. When 'the 

 elastic lungs and the muscles of the sides of the body con- 

 tract, air passes out. When the nostrils and also the opening 

 of the gullet are shut, and the floor of the mouth at the 

 same time raised, air is forced through the glottis into the 

 lungs. 



Excretory System. — The paired kidneys are elongated 

 organs situated dorsally and posteriorly in the region of the 

 urostyle. The waste-products which they filter out of the 

 blood pass backward by two ureters which open separately on 

 the dorsal wall of the cloaca, without any communication with 

 the bladder. The ureter is seen as a white line along the 

 outer side of each kidney. In the male each ureter functions 

 also as the duct of a testis. On the surface of each kidney 

 is a longitudinal yellowish streak, an adrenal gland of 

 unknown significance, and little spots mark ciliated aper- 

 tures or nephrostomes, which remain as communications 

 between the abdominal cavity and the renal veins, though 

 perhaps they are in embryonic life connected with the urinary 

 tubules. 



Reproductive System. — The males are readily distinguished 

 from the females by the swollen cushions on the innermost 

 digits of the hand, and there are some other slight external 

 differences. The breeding season begins in_ spring, and 

 then the males are often heard trumpeting to their mates. 

 The male clasps the female with his fore-limbs, and retains 

 his hold for several days, fertilising the ova as they pass out 

 into the water. 



The paired testes are oval yellowish bodies lying in front 

 of the kidneys ; the spermatozoa pass by vasa efferentia 

 through the anterior part of the kidney into the ureter or 

 Wolffian duct, which thus functions as a vas deferens. In 

 the male of R. esculenta, the vas deferens is dilated for some 

 distance after leaving the kidney ; in R. temporaria it bears 



