XXXIV. EXCRETION 



Organs of Excretion. — All the life processes which take place 

 in a living thing result ultimately in the formation of organic 

 wastes within the body. This is the direct outcome of the processes 

 we call oxidation. In the one-celled animals we find the outer 

 layer of the body used to excrete or discharge waste materials. 

 In some invertebrates, coiled tubes called nephridia are found, 

 for the purpose of throwing off nitrogenous waste. In the verte- 

 brate animals, the skin and kidneys perform this function, hence 

 they are called the organs of excretion. 



Laboratory Work on the Urino-genital System of the Frog. — The organs of 

 excretion and those of reproduction are closely connected in the frog, and 

 are best studied together. In a male frog from which the digestive tract 

 has been carefully removed, the kidneys may be found closely attached at 

 the dorsal side of the body cavity. They are red-brown in color. How 

 many are there ? What is their general shape ? The ovoid bodies, lying 

 directly on the kidneys, are the spermaries. Notice the yellowish finger- 

 like bodies just anterior to the spermaries. These are the so-called fatty 

 bodies; their function is not exactly known, though it is believed that they 

 contain a reserve supply of food. Look along the outer edge of each kidney 

 for tiny white tubes, the ureters, which connect each kidney with the cloaca. 

 Just ventral to the cloaca is the urinary bladder, a large thin-walled 

 bipartedsac; this is also connected with the cloaca. (The bladder is fre- 

 quently found in a collapsed condition. It is easy to cut it away uninten- 

 tionally) . 



Draw the urino-genital system of the male frog (twice natural size), show- 

 ing as many parts as you can. 



In the female frog, a large part of the body cavity, especially at the breed- 

 ing season, may be filled by the ovary; this contains a great number of 

 black and white eggs, which may easily be seen through its very delicate 

 walls. On each side of the body cavity, posterior to the ovary, are found 

 two long and much-twisted tubes, the oviducts. In these tubes, the eggs 

 receive a jellylike coat which protects them after they are laid. The ovi- 

 ducts are connected with the cloaca. The position of the kidneys, ureters, 

 and bladder is practically the same as in the male frog. 



Draw (twice natural size) the urino-genital system of the female frog, 

 showing the ovary and oviduct on one side removed. Label all the parts 

 shown. 



Laboratory Work on the Kidney. — An idea of the internal structure of the 

 kidney of man may be gained by examination of a sheep's kidney. Get 

 the butcher to leave the mass of fat around the kidney. Of what use might 

 this fat be ? Notice, after removing the fat, that the kidney appears to be 



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