VBETBBRATE KIDNEYS 



381 



this way each of these coiled tubes absorbs wastes either 

 from the body cavity or blood, transforms it into a secre- 

 tion, and pours it into a duct and out the pore. The action 

 is the same in every way as that of a single tubule and cap- 

 sule of the kidney. (See Fig. 171.) The kidney of the clam 

 is a coiled tube almost identical in its structural feattires 

 with this nephridium of the earthworm, while the green 

 glands of the crustacean are supposed to act in a similar 

 manner. 



Kidneys of insects. — These animals show a modification 

 of the nephridium plan in that the tubules are arranged in 

 a ring about the intestine. 

 The ends of the tubes float 

 in the body cavity of this 

 animal, and absorb the 

 waste from that, while their 

 moutlis open, not to the 

 outside, but into the in- 

 testine, so that the nitrogen 

 wastes pass out with the 

 faeces through the anus. 



Vertebrate Kidneys. — In 

 all the vertebrates we find 

 some form of kidney. These 

 kidneys differ from a nephrid- 

 ium mainly in that they 

 are a combination of many 



secreting tubules under one covering, and they differ from 

 that of man only in the manner in which the ducts termi- 

 nate. For example, the ducts of the bird and reptile kidney 

 open into an enlargement of the rectum (lower end of in- 

 testine) called the cloaca, and in these forms the anus is 





Fig. 177 — Kidney lubes of an insect; A, 

 kidney tubes; a, esophagus; b, crop; c, d, 

 stomacli; e, intestine; /, large intestine; 

 a, anus. 



