284 INVERTEBRATE PHYSIOLOGY 



say, the right than from the left? A lateral polarity is a thing quite un- 

 heard of. We must also realize that, in the case of such direct stimulation 

 by a proceeding wave, the pacemaker which has started the contraction 

 cannot exert any more influence when the wave is on the way, for then 

 the wave is out of reach of the pacemaker. The only alternative seems to be 

 the assumption of a specialized conductive system, originating in the pace- 

 maker regions, and running along the length of the heart. Such a conductive 

 system could possess polarity, as in the vertebrate heart. 



The assumption of a conductive system is strengthened by certain other 

 observations. One is inclined to pinpoint this system at the longitudinal 

 suture, along which the heart is connected with the pericardium. This 

 suture has no muscular elements and thus does not contract. As shown in 

 our laboratory with vital staining, it is a rather broad undifferentiated 

 strand of transparent material, which contains some scattered cells of 

 doubtful nature. Ebara (unpublished experiments) found the cells in the 

 suture to form a longitudinal chain. Leaving the question open whether 

 these cells represent a conductive system, we have to face the fact that, 

 according to Ebara, local damage of the suture with a needle causes the 

 peristaltic wave to stop at that point. Local damage of other parts, that is, 

 muscular parts, of the heart does not stop the propagation of the peristaltic 

 contraction. It seems clear, therefore, that the suture has some specific 

 function, that is, it has something to do with the conduction of pacemaker 

 impulses. 



We have pictured some of the major problems with which the tunicate 

 heart confronts us. Many questions still have to be solved, and suggestions 

 put forward in this paper are partly of a hypothetical nature. However, we 

 hope we have aroused interest and stimulated further research on the sub- 

 ject. A detailed review will be published elsewhere (Krijgsman, 1956). 



Summary 



Some major problems of the physiology of the tunicate heart have been 

 discussed. The tube-shaped heart possesses two pacemakers, one at each 

 end. Each of these pacemakers induces alternatively a series of peristaltic 

 pulsations. There is not sufficient evidence for either a myogenic or a 

 neurogenic nature of the pacemakers. Extrinsic regulation seems to be 

 absent. The reversal of beat is caused by the "weakness," i.e., temporary 

 exhaustion of the sensory properties of the pacemakers. The possibility 

 of the existence of a specialized conductive system is discussed. 



REFERENCES 



Bacq, Z. M., 1934a. Observations physiologiques sur le coeur, les muscles et le systeme 

 nerveux d'une ascidie (Ciona intestinalis) , Bull. Acad. Roy. Belg. CI. Set. 20, 1042. 



