202 MANUAL OF ANIMAL BIOLOGY 



already been given in previous sections to the lungs and skin and, 

 therefore, we may now consider the kidneys. (W. pp. 180-187.) 



The pair of kidneys of the Frog, or mesonephroi x as they are 

 technically termed, lie, as has been noted, in the coelom close to 

 the dorsal body wall. The microscopic study of these organs shows 

 that they consist of an enormous number of tiny, coiled, tubular 

 excreting elements, known as the uriniferous turules, which are 

 embedded in connective tissue. Each of these coiled tubes is, in 

 a general way, comparable in its structure to a single nephridium 

 of the Earthworm. In fact, considered from an evolutionary 

 standpoint, the Vertebrate kidney is generally believed to have 

 arisen by a consolidation of the nephridia-like tubules to form the 

 definite kidneys. (W. fs. 107, 132.) 



Each uriniferous tubule has at one end a capsule-like structure, 

 known as the Malpighian rody. The latter is composed of an 

 enlarged portion of the tubule (Bowman's capsule) which is lined 

 with a peculiar type of cells and encloses a greatly coiled mass of 

 capillaries, and these constitute the glomerulus. 2 The wastes 

 from the blood are given off in the glomeruli and also in certain 

 other portions of the uriniferous tubules which are invested with a 

 network of capillaries. The question as to the specific functions 

 of these two portions of the uriniferous tubules is somewhat in 

 doubt. (W. f. 131, B.) 



The uriniferous tubules of each kidney open into a common 

 collecting canal which, in turn, opens into the ureter. In the Frog, 

 there is no direct connection between the ducts from the kidneys 

 and the bladder ; all of which open directly into the cloaca. The 

 arrangement in the cloaca is such that the urine from the ureters 

 finds its way into the opening of the bladder. It collects in that 

 organ and is later expelled from the body through the cloaca. This 

 condition in the Frog is different from that found in Man and other 

 Mammals. In the latter the ureter from each kidney opens 

 directly into the bladder. A tube from the bladder, known as 

 the urethra, conveys the urine to the exterior through a separate 

 opening. 



Three types of kidneys are recognized in the vertebrate animals, 

 namely, the pronephros, mesonephros, and metanephros. The 

 structure of all these types is basically the same. That is to say 

 the functional excreting elements are nephridia-like tubules, as 



1 Singular, mesonephros. 2 Plural, glomeruli. 



