568 • Comparative Animal PhysioJooy 



Nervous regulation of heart beat has evoh'ed numerous times, sometimes 

 with augmenting fibers, sometimes with inhibiting fibers, and sometimes with 

 both. There is no evidence for nervous or hormonal control of peripheral 

 blood vessels among any invertebrate groups. Activity in cardiac nerves ap- 

 pears to be associated with liberation of chemical mediators, identification of 

 which is sure only for vertebrates. The effect of acetylcholine on a heart de- 

 pends on the innervation of the heart. 



The evolution of circulatory mechanisms shows many parallel develop- 

 ments, each tending to make a given volume of fluid serve more efficiently 

 the transport needs of the animal. Circulatory mechanisms emphasize the 

 bifurcation of the phyletic tree and the great gap between vertebrates and 

 prochordates (ascidians). 



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