SOME FEATURES OF THE PHYSIOLOGY OF THE 

 TUNICATE HEART 



B. J. Krijgsman and Nel E. Krijgsman 

 University of Cape Town 



It is well known that the heart of tunicates is an organ which, in struc- 

 ture and function, can hardly be compared with the heart of other animals. 

 In contradistinction to that of vertebrates and most molluscs it has no 

 strong muscular wall, and, contrasted with arthropods, it has no external 

 filaments by which diastolic extension is achieved. Perhaps the most strik- 

 ing feature of the tunicate heart is the remarkable reversal of beat. In other 

 groups reversal may occur occasionally and quite irregularly (Krijgsman 

 1952), but in tunicates this reversal of beat is a regular phenomenon. 



Many problems of the tunicate heart still await solution, but certain 

 points have been elucidated. We know that pulsation starts at one end and 

 proceeds along the heart as a peristaltic wave, maintained by one layer of 

 primitive muscle cells arranged nearly circularly. In spite of the fact that 

 the constriction does not really close the heart and thus a certain leakage 

 must occur, this type of propulsion gives rise to a one-way circulation as 

 long as one-way beating lasts. This has been definitely proved by v. Skram- 

 lik (1929) with injections of India ink, thereby disproving the older in- 

 vestigations of Enriques (1904), who thought that there is merely an 

 oscillation of the blood. 



Another point which seems to be quite clear is the existence of two 

 intrinsic pacemakers, one at each end of the tube-shaped heart. There are 

 plenty of arguments for this thesis. First of all one observes a series of 

 pulsations starting at one end. After some time, which may be minutes or 

 hours, pulsations start from the other end. This may result in a true compe- 

 tition, or, more usually, the active end stops. Then there is a period of rest, 

 after which the other end starts activity. This alternation in dominance 

 was observed as early as 1822 by Kuhl and v. Hasselt and confirmed by all 

 later workers. The fact that normally a pulsation starts at one of the ends 

 of the heart indicates a localization of pacemakers at those points. 



Further experiments have made it quite clear that this assumption is 

 true. Local mild stimuli, for example local heating or cooling of an end, 

 afifects the frequency and even the dominance of that end. The most con- 

 vincing experiments are probably those made by v. Skramlik (1926a,b), 

 who saw that heating of a passive end can cause it to start activity and to 

 gain dominance. Heating of an active end increases frequency, whereas 



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