72 ZOOLOGY 



and the special structure of the circulatory system depend 

 largely upon the position and degree of development of the 

 digestive and respiratory organs. In order to secure the neces- 

 sary motion of the fluids contained in the tubes, the walls of 

 the latter are supplied with muscular fibres, and contract more 

 or less rhythmically. If the motion is to have a definitely con- 

 tinuous direction, as is ordinarily the case, valves are usually 

 so placed that motion in the opposite direction will be im- 

 possible. The (one or more) contractile regions are called 

 hearts; vessels conducting blood from the heart are arteries, 

 those carrying blood toward the heart, veins. In the region 

 where the vessels are smallest and have very thin walls, the 

 exchanges between the blood and the other tissues occur. This 

 is the region of capillaries. The blood system has capillaries 

 in the walls of the digestive tract, in the respiratory organs, 

 in the kidneys, in the liver, and in and about all the tissues re- 

 ceiving a direct blood supply. The capillary region is that for 

 which the rest exists; it is the physiologically important part 

 of the system. Fig. 34 illustrates the arrangement of parts 

 found in a common type of circulatory apparatus. 



96. Demonstration. — Circulation of blood in tail of tadpole; in the web of the 

 foot of a frog; or in the fin of small fish. Distinguish veins and arteries. Notice 

 behavior of corpuscles in passing through small capillaries. Compare rate of flow 

 in vessels of diSerent size. 



97. The Excretory System and Function. — Beside the car- 

 bon dioxid eliminated from the blood in the lungs or gills, other 

 waste products of oxidation are to be removed from the tissues 

 where they are produced. Important among these are the nitrog- 

 enous wastes, urea and uric acid. In organisms in which 

 there is no regular blood system, these waste products may be 

 carried directly from the tissues to the surface by a system of 

 tubes beginning as capillaries. In the majority of animals the 

 canals (nephridia) pass from the body cavity to the exterior. 

 These are seen in a simple condition (Fig. 3 5) in the segmented 

 worms. For a more unified condition see unsegmented worms 

 (Figs. 88, 89). The kidneys of higher forms are considered 

 to be derived from these. In the higher animals the kidneys 



