250 



L. H. SNYDER AND W. J. CROZIER. 



p. 44) ; it has been noted that the number of the auriculo-ventricu- 

 lar connections of the heart may be dissimilar on the two sides 

 (Pelseneer, 1897, p. 23) ; several instances of valve-fusion seemed 

 asymmetrical (Crozier, 1919) ; and the existence of minor asym- 

 metries in the two gill series has been recorded several times 

 (Pelseneer, 1897; 1920, p. 201). But the published data fail to 

 cover the possibility of definite correlation with superior growth 

 on the right or on the left side. 



Our counts show that the distribution of asymmetries of the 

 ctenidia follows a purely random course; the absolute amount of 

 asymmetry, moreover, is small. When excess of gill-plumes on 

 the right side as compared with the left is reckoned as plus, on the 

 left side as compared with the right as minus, a frequency curve is 



32 



-5 -4 -3 -2 -I 



Variation in branch!! 

 FIG. 3. 



obtained (Fig. 3) indicating an essentially chance distribution. It 

 follows that the asymmetry is merely an accident of growth, giving 

 a measure of the independence of the two sides in forming new 

 gills. On this view, the extent of ctenidial asymmetry should 

 increase, as a rule, with size. Table II. shows that this is the case. 



