Millimeters 



Figure 219. — Auriculo-ventricular valve of C. virginica seen in longitudinal section. Auricle on the left. Bouin, 



hematoxylin-eosin. 



atism of the bivah'e heart is of diffuse nature. 

 Berths and Petitfrere (19;34a, 1934b) showed that 

 contractions of the lieart of Anodonta originate at 

 any point of tlie ventricle whether it is observed 

 in situ, or on isolated and even sectioned pieces. 

 In these studies the authors used optical methods 

 to record the beats of the hearts, which were fully 

 submerged in Ringer solution or in Anodonta blood 

 and were not stretched by writing levers. They 

 found that such distension of the ventricle removed 

 the asynchronism in automatic acti^^ty, increased 

 the amplitude of the contraction, and diminished 

 the rhythm. Jullien and Morin (1931a) reported 

 that the pulsations in dissected strips of heart 

 muscles of 0. edulis continue for some time. One 

 may conclude that the hearts of the oyster and 

 other bivalve mollusks are myogenic, i.e., their 



intrinsic automatism originates in the muscular 

 tissue. In the myogenic hearts of bivalves the 

 beat can start at any point and the contraction 

 can be local or involve the entire organ (Berthe 

 and Petitfrere, 1934b). This type of activity 

 differs from that of the neurogenic hearts, such as 

 those of arthropods, in which the excitation wave 

 of the beat originates from the nerve cells of the 

 ganglia. 



THE PACEMAKER SYSTEM 



We know that the rhythmic activity of the 

 hearts of bivalves originates in the heart itself and 

 is not provoked by impulses from the central 

 nervous system. Whether this automatism is 

 produced by localized pacemakers or is a general 

 property of all muscle fibers has not been adequate- 



CIRCULATORY SYSTEM AND BLOOD 



245 



