8 GENERAL ZOOLOGY 



(Fig. 3), which extends through the abdomen just beneath 

 the dorsal surface. On account of its position in the body 

 it is often spoken of as the dorsal vessel. The heart is pro- 

 longed anteriorly into a tube leading to the head, and is 

 partially divided by valves into eight chambers, which per- 

 mit the movement of the blood only from the posterior to 

 the anterior end. When the blood has been passed out into 

 the general body cavity, it returns through a closed tube 

 (ventral sinus) between the muscle masses lying in the lower 

 (ventral) part of the body, and reenters the heart through 

 side openings. 



The Respiratory System. The function of the respiratory 

 system is to provide for a constant supply of oxygen for all 

 the organs of the body and to remove waste material, chiefly 

 carbon dioxide. This is accomplished by a system of tubes, 

 called tracheae, communicating with the surface by the spir- 

 acles of the thorax and abdomen. The tracheae are con- 

 nected and form a network of tubes running to all parts of 

 the body, even out into the legs and wings. They are also 

 in connection with a system of large air sacs (Fig. 3) ex- 

 tending through the body. The tracheae are kept perma- 

 nently open by a spiral thickening of their chitinous lining, 

 so that air may enter freely at all times. Air is drawn into 

 and forced out of the tracheae by rhythmic contractions of 

 the body. A constant supply of oxygen is thus assured and 

 carbon dioxide is expelled. The completeness of the respir- 

 atory system in the grasshopper is in striking contrast to 

 the undeveloped character of the circulatory system. 



The Excretory System. The union of the oxygen taken in 

 during respiration with the carbon in the body produces car- 

 bon dioxide, — a waste product. This leads us to consider 

 the organs which assist in the removal of materials which 

 have helped to build up the body substance and have be- 

 come so changed chemically that they are no longer useful. 



