Circidation of Body Fluids 567 



Unio.'^''- "" In OctopMs/"'- "'* also, a shock applied during systole may 

 nroionp or heighten the contraction, extrasvstoles are not followed bv a com- 

 pensatory pause, and the heart can be smoothly tetanized. Anodal polariza- 

 tion slows or stops hearts of Leptodora, Daphnia,^''-' and Ahtrfx'"^ in diastole 

 like the hearts of vertebrates. 



In Liniuhis the cardiac ganglion contains two types of nerve fiber, large 

 fibers which show a refractory period of 4-6 msec, and smaller fibers with a 

 refractory period of 500-600 msec.^'^ 



The differences between arthropod-mollusc hearts and vertebrates hearts in 

 response to electrical stimulation and in all-or-none properties may be due to 

 differences in the syncytial nature of the heart muscle. The histological cor- 

 relations with response are poorly known, and research should be extended 

 to additional animal groups. 



CONCLUSIONS 



To be of adaptive value a circulatory system must respond to stress; blood 

 must be available where it is needed in adequate amounts when required. 

 Circulatory mechanisms differ greatly in their efficiency of response to stress. 

 Open svstems— coelom, pseudocoel, or hemocoel— without hearts are adequate 

 where respiratory and nutritional demands are not great. Such pools of fluid 

 can more adequately distribute oxygen than acquire it. With increase in body 

 size and activity hearts are added to some open systems, as in numerous crus- 

 taceans and molluscs. These hearts are incapable of developing high pres- 

 sures, however, and circulatory flow is largely accomplished by the tone of 

 body muscles. The blood volume is large and the transport of oxygen from 

 gills to muscle is relatively slow. Some annelids have in addition a system 

 of closed blood vessels, particularly for oxygen transfer, but the coelom re- 

 mains the main circulatory mechanism for excretion and nutrition. The in- 

 efficiency of the open system is solved in insects by reduction of the hemocoel 

 to minor functions and by the carrying of air directly in tubes to the muscles; 

 this permits high activity in a compact body. In the cephalopods and verte- 

 brates part of the circulatory system became closed off in vessels, with the re- 

 suit that systemic blood pressure is high, velocity of blood flow is also high, 

 and blood volume is low (not known for cephalopods). The lower verte- 

 brates retained a sort of open system, the lymph system, in which flow is aided 

 by accessory lymph hearts; in higher vertebrates the lymphatics are of much 

 less importance. Among vertebrates there are complex reflex and hormonal 

 systems for the maintenance of blood pressure; the extent of similar regulating 

 systems among invertebrates is unknown. 



Heart muscle in all animal groups has some properties of visceral and some 

 properties of somatic muscle. The ability of a heart as a whole to contract in 

 all-or-none fashion may be related to the degree of its syncytial structure. 

 Vertebrate hearts differ strikingly in this respect from mollusc-arthropod 

 hearts. The property of rhythmic activity is inherent in many nerve cells and 

 visceral muscles. It is not surprising, then, that cardiac pacemakers are di- 

 verse and it is possible that the causative chemical rhythms are similar in all 

 pacemakers. Myogenicity, which is more primitive, is retained in adult ver- 

 tebrates and molluscs but is replaced by neurogenicity in most adult arthro- 

 pods. 



