58 THE ELEMENTARY NERVOUS SYSTEM 



visceral beats predominate. There seems to be no ques- 

 tion but that in the heart of the tunicate there are two 

 centers from which contractions may arise, one at the 

 advisceral and the other at the abvisceral end. It is ap- 

 parently the alternate control of the heart by first one and 

 then the other of these centers that brings about the re- 

 versal in the activity of this organ (Loeb, 1899, 1902). 



Whether these two centers are located in the muscle of 

 the tunicate heart or in other tissue connected with this 

 muscle is somewhat uncertain. The heart itself is com- 

 monly described as made up of a single layer of muscle 

 cells whose inner face is covered with a delicate endo- 

 thelium. Nerve cells have been supposed by most investi- 

 gators to be entirely absent, but Hunter (1902) claimed 

 to have identified them in Molgula, where they are said to 

 be especially abundant at the two poles of the heart, the 

 regions from which the contraction waves start. Their 

 presence is also mentioned by Alexandrowicz (1913). 

 The observations that the isolated middle portion of the 

 heart, where nerve cells are very sparse, if in fact they 

 are present at all, will continue in rhythmic contraction 

 (Schultze, 1901; Bancroft and Esterly, 1903; Hecht, 

 1918) even after it has been isolated and placed in sea- 

 water suggest that these cells have no more to do with 

 originating this form of heart-beat than the similar cells 

 in the vertebrate heart have. Hecht (1918) has also 

 pointed out that in perfectly normal specimens of Ascidia 

 the contraction wave in the heart muscle may sometimes 

 originate in this middle portion of the heart, showing 

 that this part may normally act as a pacemaker for the 

 whole organ and that its action in isolation is, therefore, 

 nothing unusual. It has also been determined that the 

 rate of transmission of the contraction wave over the 



