THE TUNICATE HEART 279 



existence of nerve elements in the tunicate heart. Although it may well be 

 that technical difficulties, for example refractory behavior of possible nerve 

 elements, are responsible for negative results, the histological data avail- 

 able do not convince us, at the moment, that the pacemakers have a neuro- 

 genic character. 



The same uncertainty applies to electrical stimulation of the heart. Most 

 of the experiments were made by early workers. The data obtained do not 

 help us, for they could refer equally well to either a neurogenic or a myo- 

 genic pacemaker. 



Some data are available on the application of drugs on the tunicate 

 heart. Unfortunately these results are confusing. Bacq (1934a,b, 1935), 

 for instance, found no influence of acetylcholine or adrenaline on the heart 

 of Ciona. Ebara (1953) and Waterman (1939, 1942, 1943) have detected 

 an influence, but their results are not exactly compatible. Results obtained 

 by the application of other drugs add to the confusion. We feel that much 

 more work must be done along these lines before we can draw definite 

 conclusions. The existing confusion may have arisen partly from the fact 

 that the different aspects of the activity of the heart, that is, frequency, 

 length of pulsation period, and duration of pause between the pulsation 

 periods, have not been given due consideration as separate phenomena. 

 A depressive drug, for example, first depressing a more sensitive center, 

 might lead to longer pulsation series at the other one, thus wrongly sug- 

 gesting a stimulation of the latter. As a matter of fact, the factors concerned 

 cannot be studied independently if one center can obscure the response of 

 the other. One therefore should work with hearts ligatured in the middle. 

 Length of pause and duration of pulsation series could then no longer be 

 obscured by competition. Such studies are in progress in our laboratory 

 and seem to indicate a m}'Ogenic nature of the pacemakers of the Ciona 

 heart. However, we want to express some reserve, since these experiments 

 are not sufficiently advanced for a definite opinion. 



All in all, we are reluctant to call the pacemakers of the tunicate heart 

 either myogenic or neurogenic. One has to wait for more definite results 

 in the field of histology and pharmacology. 



The next problem which we wish to consider is the possible existence 

 of extracardiac regulation. We can be brief on this point. Some fifty years 

 ago Hunter (1903) claimed that he saw nervous connections between the 

 pacemakers of the Molgula heart and the central nervous system. He alone 

 is of this opinion, for no one else has ever seen extracardiac nerves in 

 tunicates. The fact that some workers have claimed that the destruction of 

 the central nervous system afifects the heart beat must not be given too 

 much weight, for several investigators have shown that all kinds of dam- 

 age influence heart beat. Elimination of the central nervous system does 



