Millimeters 



Figure 216. — Cross section through auriculo-ventricular 

 septum of C. virginica. FormaHn 5 percent, hema- 

 toxj-lin-eosin. 



inspection. The electrocardiogram of the oyster 

 heart {0. edulis) published by Eiger (1913) shows 

 that the interval is about 0.5 second. A similar 

 condition in the heart of C. virginica was demon- 

 strated on an electrocardiogram (fig. 222) made 

 in the Bm'eau's shellfisii laboratory by removing 

 part of one valve and placing the electrodes on 

 the pericardium wall and on the adjacent tissues. 

 Action currents observed by Taylor and Walzl 

 (1941) in the ventricle of the excised heart of 

 C. virginica consist, according to their interpre- 

 tation, of two components, a major diphasic 

 wave preceding the contraction, and a slow wave 

 at the time of contraction. 



The refilling of the heart during the diastolic 

 phase is dependent on pressure mechanism in the 

 pericardiimi. Krijgsman and Divaris (1955) pro- 

 pose the following probable explanation which 

 requu-es further corroboration. Tlie change in the 

 hydrostatic pressure in the pericardial chamber, 

 caused by systolic contraction, is compensated by 

 the expansion of the auricles. At the moment the 

 ventricle starts to contract it exerts a suction 

 which brings in blood through the reno-pericardial 

 canal and venous sj-stem. Thus, the contraction 



Microns 



200 



Figure 217. — Cro.ss section of the wall of the auricle of 

 C. virginica. Outside wall is covered with glandular 

 epithelium. Rouin, hematoxylin-eosin. 



of the ventricle automatically results in the expan- 

 sion of the auricles. This interesting hypothesis 

 may be corroborated by observations on hydro- 

 static pressure inside the heart and in the peri- 

 cardial cavity and by motion pictures of the 

 sequences of ventricular and auricular ])eat. To 

 my knowledge these have not yet been made. 



Observations on bivalve hearts in situ show 

 that the ventricle and auricles alternately increase 

 in size while they are being filled with blood. 

 Both auricles of the oyster heart contract simul- 

 taneously (Skramlik, 1929). 



Experimental e\'idence indicates that the autom- 



0m^^ 











Millimeters 



100 



Figure 218. — Cross section of the heart at the auriculo- 

 ventricular valves of C. virginica. Bouin, hematoxylin- 

 eosin. 



244 



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