Circulation of Body Fluids 



555 



and slow diastolic portions (Fig. 202)."-'' The electrocardiogram of the 

 oyster^^'** and of fresh-water mussels (Fig. 203) consists normally of a diphasic 

 component near the beginning of contraction and a slow wave associated with 

 contraction. The interpretation of the rapid and slow components in the 

 molluscan electrocardiogram is uncertain, but the fast wave probably repre- 

 sents spread of excitation. There is some variability in shape, according to 

 electrode placement. 



The neurogenic hearts of arthropods show very different electrocardio- 

 grams; there may be fast and slow deflections like those of the myogenic 

 hearts, but in addition the arthropod electrocardiogram is typically oscilla- 

 tory. The large waves have sometimes been designated by letters, correspond- 

 ing to the terminology of the waves in the vertebrate ECG.''-' This practice 



Fig. 202. Electrocardiogram from Helix pomatia. From Arvanitaki and Cardot.' 



Fig. 203. Electrocardiogram of fresh-water mussel. 



Y / 



Fig. 204. Electrocardiogram from Astacus. From Hoffman."" 



is misleading and erroneous, inasmuch as the nature of these deflections, is 

 very different in the two animal groups. In Figs. 204 and 205 are shown 

 some representative patterns from hearts of arthropods, Astacus^'*' ■'- and 

 grasshopper.^" Simliar oscillatory waves were recorded from hearts of larvae 

 of Galleria^^'^ and Dytiscus.^^ The pattern of the ECG in arthropods varies 

 greatly according to electrode position, temperature, and other factors. In 

 the grasshopper Melanoplus, for example, treatment with cold saline tends 

 to suppress the oscillations and to emphasize the large fast and slow waves 

 (Fig. 205).-*'' 



It is probable that the large deflections of the arthropod ECG represent 

 muscular activity in which repolarization is slow. Such complex action po- 

 tentials are frequently recorded from sheets of short muscle fibers, as in ver- 

 tebrate smooth muscle (Ch. 16, p. 591)- The oscillations in arthropod elec- 



