4 00 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



In all animals violent motion causes increased respi- 

 ration, but the mechanism may be different. In verte- 



Fig. 278. — Side view of the circulatory and respiratory system of a crawfish : 

 h, heart ; gg, the gills. The arrows show the course of the blood. 



brates increased motion causes, of course, increased 

 waste in the blood. This in time stimulates the respira- 

 tory centers (medulla), and determines through the 

 pneumogastric nerve increased action of the respiratory 

 muscles. In crustaceans the increased motion produces 

 increased respiration not only through the mediation of 

 the nervous system, but also directly by the motion of 

 the gills themselves. 



Circulation. — In the dorsal region behind the middle, 

 immediately beneath the carapace and between the points 



of the gills, is seen 

 the heart (Zf, Fig. 

 276), a large or- 

 gan whose pulsa- 

 tions throw the 

 blood fore and 



Fig. 279.— Diagram showing the general course of aft tnrou gh the 

 the circulation in crustaceans: G, gills; H, large dorsal arte- 

 ries (3 a) to the 

 tissues. Another artery goes from the heart downward 

 to form the sternal artery, and thence to the limbs and 

 lower part of the body (Fig. 277). From the tissues the 



