REPRODUCTIVE SYSTEM. 547 



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 nostrils and 

 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. When the pressure on the lungs is 

 relaxed, and when the muscles of the sides of the body 

 contract, the air passes out. 



Excretory system. — The paired kidneys are elongated 

 organs situated dorsally and posteriorly beside 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, and are not directly connected 

 with the bladder. The ureter or Wolffian duct is seen as a 

 white line along the outer side of each kidney ; in the male 

 it functions also as the duct of the testis. On the ventral 

 surface of each kidney is a longitudinal yellowish streak, 

 which is an adrenal gland of unknown significance, and 

 little spots mark ciliated apertures or nephrostomes, which 

 remain as communications between the abdominal cavity 

 and the renal veins, though they are originally connected 

 with the urinary tubules. There are also, as in higher 

 Vertebrates, openings from the abdominal cavity into the 

 lymphatic system. 



Reproductive system. — The males are distinguishable 

 from the females by the swollen cushions on the first fingers, 

 and by some other external differences. The breeding 

 season begins in spring, when the males trumpet 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 Wolffian 

 duct, which functions both as a ureter and as a vas deferens. 

 In the male of R. esculenta the vas deferens is dilated for 

 some distance after leaving the kidney; in R. temporaries 



