THE FROG 199 



form the termination of the sensory nerves. They are of different 

 kinds, and doubtless have different functions, but all are associ- 

 ated with what is in general called the sense of feeling or touch. 



The Excretory Organs. Lying in the back part of the ab- 

 domen near the legs are two flat, rather oval bodies, one on 

 either side of the middle line, the kidneys; Fig. 92. Each is 

 abundantly supplied with blood vessels, a fact which indicates 

 important functions. Microscopic study shows them to be made 

 of many coiled tubes, similar to the nephridia of the earth- 

 worm. These tubes remove excreted products from the blood 

 which passes through them. From the outer side of each a 

 small duct, the ureter, passes backward toward the cloaca, 

 where it empties into the bladder (Fig. 90 bl), a rather large two- 

 lobed sac, which may be filled with the urine secreted by the 

 kidney, or may collapse when empty. It opens into the cloacal 

 chamber, and its contents are discharged through the common 

 cloacal opening. (In man a special duct, the urethra, leads 

 from the bladder to the exterior.) 



Reproductive Organs. The two sexes in the frog are in 

 separate individuals, thus differing from the condition found 

 in the earthworm. The male may be distinguished externally 

 by a thick pad on the under side of its thumb, which is rather 

 large in the spring, but small at other seasons of the year. 

 The spermaries are found in the male at the upper end of the 

 kidneys; Fig. 92 sp. They are two in number, rounded or 

 oval in shape, and of a light yellowish color. Attached to them 

 are usually several branching masses of yellow fat. The sperm 

 produced in the spermaries are carried through some delicate 

 ducts into the kidney. These ducts, the vasa efferentia, pass 

 through the kidneys and empty into the ureters, which lie 

 on their outer edge. The ureters in the frog thus serve for the 

 exit of both the kidney secretion and the secretions from the 

 spermaries. These ureters are, in some species of frogs, en- 

 larged into a small sac just at the point where they enter the 

 cloacal chamber, and in these sacs the sperms are stored until 



