PRINCIPLES OF PHYSIOLOGY 93 



cell by osmosis because the concentration of salts is greater in the cell 

 than in the surrounding environment. These forms have evolved a 

 contractile vacuole, which fills with fluid from the surrounding proto- 

 plasm and then empties to the exterior. Sponges and coelenterates have 

 no specialized excretory organs and their wastes simply diffuse from the 

 intracellular fluid to the external environment. 



The simplest animals with specialized excretory organs are the flat- 

 worms and nemerteans, which have flame cells (Fig. 5.6) equipped with 

 flagella, and a branching system of excretory ducts from the flame cells 

 to the outside. The flame cells lie in the fluid which bathes the cells of 

 the body, and wastes diffuse into the flame cells and thence into the 

 excretory ducts. The beating of the flagella (which suggests a flickering 

 flame when seen under the microscope) presumably moves fluid in the 

 ducts out through the excretory pores and thus aids diffusion. As in 

 the contractile vacuoles of the protozoa, the chief role of the flame cells 

 is probably the regulation of the w'ater content of the animals. Some of 

 the metabolic wastes are removed by diffusion through the lining of the 

 gastrovascular cavity. 



Each segment of the body of an earthworm contains a pair of 

 specialized excretory organs known as nephrldia. A nephridium is a 

 long, coiled tubule, opening at one end to the body cavity in a funnel- 

 shaped structure lined with cilia, and at the other end to the outside of 

 the body via an excretory pore. Fluid is moved through the nephridium 

 in part by the beating of its cilia and in part by the contraction of 

 muscles in its wall. The earthworm excretes a very dilute, copious urine, 

 at a rate of about 60 per cent of its total body weight each day. 



The crustacean excretory organs are the green glands, a pair of 

 large structures located at the base of the antennae and supplied with 

 blood vessels. Each gland consists of three parts: a coelomic sac, a green- 

 ish, glandular chamber with folded walls, and a canal which leads to a 

 muscular bladder. \Vastes pass from the blood to the coelomic sac and 

 glandular chamber; the fluid in them is isotonic with the blood. Urine 

 collects in the bladder and then is voided to the outside through a pore 

 at the base of the antenna. 



The excretory organs of insects, the malpighian tubules, are quite 

 different from those of the crustaceans. They lie within the body cavity 

 (hemocoel) and empty into the digestive tract. \Vastes diffuse into these 

 tubules and are excreted into the cavity of the digestive tract. 



The kidneys, the vertebrate excretory organs, remove wastes from 

 the blood and regulate its content of water, salts and organic substances. 

 The structural and functional unit of the kidney is the kidney tubule 

 (Fig. 5.6). This is in close contact with the blood stream, for a tuft of 

 capillaries projects into the funnel-shaped Bowman's capsule at the end 

 of the tubule. The tubule may be quite long and looped, and in contact 

 with additional capillaries along its length. It eventually opens to the 

 outside of the body via collecting ducts and other intermediate tubes. 

 Substances are filtered into the kidney tubules from the blood capillaries 

 in the Bowman's capsules. Then, some substances are reabsorbed into 

 the blood stream and others are secreted from the blood into the urnie 



