THE CIECULATOBY SYSTEM. 107 



VII. THE CIRCULATORY SYSTEM. 



The liquid medium that distributes the digested food from the 

 alimentary canal to the cells of the body tissues is called the hlood, 

 and the contractile organ that keeps the blood in motion is the heart. 

 In vertebrate animals the blood is contained entirely within tubes 

 called arteries and veins, but in insects and most other invertebrate 

 animals the blood simply fills the empty spaces between the viscera 

 of the body-cavity, which spaces may, however, constitute definite 

 channels or sinuses, and may even be shut in by special membranes. 

 Besides carrying and distributing the digested food that is absorbed 

 into it in solution, the blood of animals generally has also to dis- 

 tribute oxygen to the tissue cells and carry off their waste products. 

 Oxygen is obtained from the air and, like any other gas, is soluble 

 in liquids. Hence it is present in the blood not in the form of small 

 bubbles of gas but in solution, just as it is in all water exposed to the 

 air. The respiratory system (see page 116) is simply a special con- 

 trivance for bringing air into close proximity to the blood so that 

 its gases may diffuse into the latter, but many soft-bodied animals 

 like earthworms absorb air directly through the skin. Vertebrate 

 animals have a substance in their blood called hemoglobin which is 

 contained in the red corpuscles and has a great capacity for absorb- 

 ing oxygen. It, therefore, enables the blood to carry much more of 

 this gas than could be dissolved simply in its plasma. Invertebrate 

 animals do not need so much oxygen as vertebrates, and, therefore, 

 most of them can get along with that which dissolves in the color- 

 less blood plasma without the special aid of hemoglobin. Most 

 insects, however, being excessively active creatures, must have a 

 rapid metabolism in their cell tissues, and consequently they need 

 much oxygen to consume the product of this metabolism, but they 

 belong to the class of animals without red blood and, hence, nature 

 has provided them with another means of obtaining a special supply 

 of air, namely, a set of air-tubes branching minutely over nearly all 

 the internal organs, the tissues, and even most of the cells in the 

 body. (See " The Respiratory System," page 112, for discussion of 

 oxidation and removal of waste products.) 



The blood of insects is usually a colorless liquid containing opaque 

 granular cells or corpuscles floating in it. There are no special blood 

 vessels, but there are very definite channels between the muscles and 

 viscera through which the blood flows, while conspicuous membranes 

 stretched across the dorsal and ventral walls of the abdomen (fig. 1, 

 DDph and VDph) inclose special dorsal and ventral sinuses which 

 play an important part in the circulation. These membranes, called 

 diaphragms, are rhythmically contractile, and contribute much to 



