^„ 60 



Microns 



Figure 220. — Nerves in the heart muscle of C. trirginica vitally stained in methylene blue. Glycerin-jelly. 



ly studied. The presence of nerve ceUs in the heart 

 has been confirmed for many bivalves, gastropods, 

 nudibranchs, and cephalopods (Dogiel, 1877; 

 Suzuki, 1934a, 1934b; Dubuisson, 1933). On the 

 other hand several investigators deny the presence 

 of nerve cells in the heart of moUusks and consider 

 that connective tissue cells were mistakenly 

 described as nerve cells (Krijgsman and Divaris, 

 1955). Motley (1933), Esser (1934), and Prosser 

 (1940, 1942) were unable to find them in Anodonta 

 and VenuK. Inconsistencies in the results are 

 probably due to the uncertainties encountered in 

 staining nervous elements of the heart with the 

 usual histological technique and frequent failures 

 in using some brands of methylene blue. 



It is known that in Anodonta and Mytilus the 

 wave of ventricular contraction starts at the 



posterior end. Furthermore, by applying heating 

 to various places of the hearts of Anodonta, Unio, 

 and Mytilus DeBocr (1929) was able to^how that 

 warming the posterior part of the ventricle 

 increases the beat frequency, whereas the heating 

 of the anterior part has no effect (Krijgsman and 

 Divaris, 1955). In the heart of a dying oyster 

 (0. edulvi), the aortic region continues to beat for 

 a longer time than do the other parts of the organ; 

 the isolated hearts seldom beat if the aorta is 

 completely cut off from the preparation (JuUien 

 and Morin, 1931a). This is also true for the 

 longitudinal fragments of the heart, which con- 

 tinue to beat if they contain a piece of aorta. 

 These observations seem to support the opinion 

 that in most cases the bivalve heart possesses a 

 diffuse myogenic pacemaker. 



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FISH AND WILDLIFE SERVICE 



