64 ZOOLOGY. 



nated by the same organ which admits the oxygen, inasmuch 

 as the entrance of one gas is not retarded by the outward pass- 

 age of the other. This double process constitutes respiration, 

 although the latter half is also appropriately described as ex- 

 cretory. The surface devoted to the exchange of the gases- 

 and the special devices necessary to renew the air or water 

 make up the respiratory system. The respiratory organs are 

 frequently associated with the anterior or posterior end of the 

 digestive tract. As in the case of other foods, the blood is the 

 vehicle by which oxygen is distributed from the gills or lungs 

 to the parts of the body needing it. 



92. The Circulatory System and Function. In such con- 

 ditions as are shown in Fig. 79, there is no circulatory system. 

 The digested food is merely distributed from cell to cell. In 

 animals in which the digestive apparatus is well developed, 

 some device becomes necessary for the distribution of the 

 food. The body cavity with its contained fluids may do this 

 work as in Fig. 29. Usually however when the mesodermal 

 layers become well developed, there arises therefrom a series 

 of branching tubes containing special fluids, blood or lymph. 

 These tubes by their ramifications connect the digestive sur- 

 faces with the various parts of the body. Some branches like- 

 wise extend to those special surfaces where the oxygen of the 

 external medium may be had. Naturally then the complexity 

 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 to\vard the heart, veins. In the region 

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



