XIII.] THE FROG. 175 



shut, and it is said that frogs may be asphyxiated by keeping 

 their mouths open. 



In addition to its principal pulmonary apparatus of re- 

 spiration, the Frog has a secondary respiratory apparatus in 

 its moist and delicate skin. A considerable amount of venous 

 blood is, in fact, constantly supplied to this organ by the large 

 cutaneous branch of the pulmo-cutaneous artery. It has been 

 experimentally ascertained that frogs in which the lungs 

 have been extirpated will continue to live and respire for a 

 considerable time, especially at a low temperature, by means 

 of the skin. 



The kidneys are elongated and flattened from side to side, 

 and are kept in their places by the continuation of the peri- 

 toneum over their ventral faces. The ducts of the kidneys 

 pass from about the junction of the middle and posterior thirds 

 of the outer edge of each kidney and, approaching as they pass 

 backwards, open by two small closely approximated slit-like 

 apertures in the posterior wall of the cloaca. 



The urinary bladder is a large bilobed sac, opening poste- 

 riorly, by a wide median aperture, into the anterior end of the 

 cloaca, on the ventral side of the rectum. 



The testes are spheroidal yellowish bodies situated in 

 front of the kidneys and enveloped in peritoneum, a fold of 

 which, forming a sort of testicular mesentery or mesorchium, 

 passes into that which covers the ventral face of the kidney. 

 The delicate vasa efferentia of the testes may be seen travers- 

 ing this fold to enter the substance of the kidney. They 

 communicate with the urinary tubules, and thus the duct of 

 the kidney serves not only as the duct of the urinary excretion 

 but as the vas deferens. 



The spermatozoa of Rana esculenta have thick and cylin- 

 drical heads, while those of Rana temporaria are linear. 



The ovaria are broad lamellar organs, very large and 



