DISPOSAL OF WASTES 



135 



capillaries 

 around tubules - 



bladder 



blood 

 vessels 



somewhat stellate or irregular in shape, hollowed out to form a funnel- 

 shaped cavity within itself. A number of long, slender cilia (the ' ' flame ' ' ) 

 take their origin from the body of the cell and hang freely into the funnel- 

 shaped cavity. In life, the cilia beat continuously and by their beating 

 cause currents in the liquid wliich is excreted into the funnel by the cell. 



Nephridia. — In the annelid worms each segment or somite (with some 

 exceptions) is provided with a pair of more or less coiled tubes, the 

 nephridia, which have a ciliated opening, the funnel or nephrostome, 

 which projects through the septum into the cavity of the somite ahead. 

 There it opens directly into the body cavity or coelom. The other end of 

 the coiled tube is connected to the body 

 wall where it has an opening to the exterior, 

 through the nephridiopore (Fig. 109). 

 Through much of its course this tube is 

 surrounded by a network of capillaries, a 

 feature of the excretory organ of all the 

 higher animals. In its operation, the 

 nephridium takes in fluid from the coelom 

 through the nephrostome. This fluid con- 

 tains wastes exuded into it by the various 

 tissues, but it also contains some usable 

 substances, one of them being glucose. As 

 the fluid passes along the tube, the glucose 

 and other useful substances are absorbed by the tubule walls and are 

 carried away in the capillaries to be used elsewhere. Excess water is 

 also thus reabsorbed into the blood, and the fluid finally ejected at the 

 nephridiopore is highly concentrated. 



Kidneys. — In embryos of the higher animals the excretory system 

 starts in a form which is comparable to a row of nephridia in the earth- 

 worm. It consists of a series of uriniferous tubules, a pair in each segment, 

 the inner ends of which open into the coelom. The outer ends, instead 

 of opening to the outside independently, all empty into a pair of tubes, 

 one on each side, and these open to the exterior. In the course of develop- 

 ment the coelomic openings, with a small portion of the tube, are closed 

 off. Minute networks of blood capillaries are pushed into the sides of the 

 tubules near the coelomic ends, and in the adult organ the tubule ends 

 at that point. The tubule wall has grown almost completely around the 

 invading group of capillaries, to form a double-walled cup through the 

 open interior of which a blood vessel passes. This cup and the blood 

 vessels in it are together known as the renal corpuscle (Fig. 110). The 

 ^\alls of the cup are Bowmaii's capsule, and the contained blood vessels 

 are the glomerulus. The renal corpuscles with the uriniferous tubules 

 are the essential excretory units in the vertebrate animals generally. 



nephridiopore 



Fig. 109. — Nephridium of 

 earthworm. (From Storer, "Gen- 

 eral Zoology.") 



