THE TUNICATE HEART 283 



there is evidence that this release is based on chemoreception of a meta- 

 bolite (Krijgsman and Divaris, 1955), but nothing is known about pos- 

 sible chemical agents in tunicates. However, if we accept the principle as 

 a reasonable one, we can develop a preliminary picture of the pacemaker 

 activity. The threshold for a possible metabolite might rise during spon- 

 taneous activity until the pacemaker no longer responds to it. This, then, 

 would be the "fatigue." During the subsequent rest the threshold could 

 decrease until the metabolite again acts as a stimulus. Once we accept the 

 probability that pacemakers are some sort of chemoreceptors, this line of 

 thought seems adequate. Various sense organs show similar types of 

 fatigue or adaptation with subsequent restoration. Hypothetical as this 

 explanation may be, no other comprehensible picture of the periodic 

 reversal of beat of the tunicate heart can be offered at the moment. It can 

 only be determined by future work. 



We do not know whether there is any significance in the periodic re- 

 versal of beat. One can see no reason why a permanent one-way circulation 

 should not be efficient. Presumably the cause of the reversal is the weak- 

 ness, that is, the periodic exhaustion of the sensory mechanism of the pace- 

 makers, which has no physiological significance. 



Finally we must touch upon the possibility of the presence of a con- 

 ductive system in the tunicate heart. One is inclined to think in terms of 

 certain stimuli of a more or less general nature, directly resulting from the 

 contracting pacemaker, spreading into the adjacent region, thus evoking 

 contraction of that area, and so on. Such stimuli might be, for example, 

 the production of a potential difference, the release of ions or of a certain 

 metabolite. Since the muscle fibers are circularly or spirally arranged and 

 closely packed, such stimuli cannot be expected to be released by the ends 

 of the muscle fibers only. On the contrary, one might expect that the stimu- 

 lating factors impregnate adjacent regions by traveling in the wall parallel 

 to the longitudinal axis of the heart. Thus they should stimulate the 

 muscle fibers not in a polar but in a lateral way. 



Unfortunately there are certain facts which militate against the accepta- 

 bility of this point of view. We shall mention some of the most pregnant 

 arguments. 



Some workers, for example Hecht (1918), v. Skramlik (1926a, 1930a), 

 Ebara (1954), and ourselves (unpublished experiments), have found that 

 the velocity of the contraction waves in Ascidia, Ciona, and Polycitor is not 

 the same in both directions. The figures obtained are significant and quite 

 convincing. But how can we understand these findings without postulat- 

 ing the existence of a special conductive pathway? If contracting fibers 

 could stimulate neighboring fibers directly — and most likely laterally — 

 why then should the muscle fibers respond more slowly when struck from. 



