278 INVERTEBRATE PHYSIOLOGY 



cooling may stop it. Heating or cooling of other parts of the heart only 

 cause a change in amplitude. These results are similar to those obtained 

 with the vertebrate heart ; that is, local change in temperature of the pace- 

 maker afifects the frequency of the beat, but changes in other parts aflfect 

 only the amplitude. This has been fully confirmed in our laboratory with 

 Ciona (unpublished experiments). 



Further proof for the peripheral position of the pacemakers has been 

 obtained by ligaturing or cutting the heart in the middle. Contractions then 

 start at each end. Finally we may mention the destruction of one end, 

 which results in waves originating from the other end only (see Krijgs- 

 man, in press). 



This well-known experimental work clearly proves the presence of 

 localized pacemakers at each end of the heart. However, other portions 

 of the heart can also show automatic activity ; for, if the peripheral pace- 

 makers are removed, the isolated central part can take up a slow beating 

 after some time. Even small fragments of the central parts may .show 

 rhythmic contractions. 



It therefore seems clear that all parts of the tunicate heart have the 

 power of automatism. In normal circumstances these basic diffuse auto- 

 matic properties are overruled by the centers at the ends, which induce 

 their rhythm on the whole heart by their more powerful automatism. 



We cannot agree with v. Skramlik (1926a,b, 1929, 1930b, 1933, 1938, 

 1941), who speaks of a third center of automatism in the middle of the 

 heart. Although in abnormal circumstances one often observes peristaltic 

 waves originating in the central region, we are reluctant to call this a pace- 

 maker in the true physiological sense. After all, the peripheral pacemakers 

 always dominate under normal conditions ; the so-called central center 

 does not appear to have any functional significance. 



Whereas the points thus far discussed have been well established, there 

 are other major problems which still remain unsolved. Such problems are, 

 for example: (1) Has the heart myogenic or neurogenic pacemakers? 

 (2) Is there an extrinsic regulation? (3) What is the cause of reversal? 

 (4) Is there a specialized conductive system which propagates the stimuli 

 produced by the pacemakers ? 



The answer to the question whether the pacemakers are of the neuro- 

 genic or of the myogenic type must be that we do not have sufficient evi- 

 dence for either of these two possibilities. First of all, histological evi- 

 dence is contradictory. Hunter (1902) was quite definite in stating that 

 each end of the heart of Molgula possesses a ganglion consisting of a small 

 number of nerve cells. Unfortunately his pictures are not convincing. 

 Millar (1952), working with Ciona, found similar cells, but he is con- 

 vinced that they are connective-tissue cells. Other workers also deny the 



