Compensatory Regulation. 



33 



ined for the distribution of opercula. By far the most extensive 

 observations are on Hydroides dianthus. 



Table HI. Hydroides dianthus. Position of Opercula. 



Explanation of Table. 

 F = Functional operculum; R = Rudimentary operculum. 



*These two specimens had a rudimentary operculum on each side. 



•j-These irregulars come under four heads: i. One specimen with Right = operculum missing; Left = 

 operculum between rudimentary and functional stage. 2. One specimen with Right = between rudi- 

 mentary and functional; Left = rudimentary operculum. 3. Two specimens with Right = functional 

 operculum; Left = two-thirds developed functional. 4. One specimen with Right = rudimentary; 

 Left = one-half developed functional. 



Jin this case the number of unclassified irregular cases was unfortunately not put down. My notes 

 give only the indefinite statement "several abnormal and incomplete ones observed are not included in 

 the present list." Assuming that the percentage of such cases is the same as in the other two groups, 

 where it is 5.4 per cent of the total number, we will not be far astray in making the unknown number 

 equal to seven. 



The relative relation betv^een right-handed and left-handed 

 forms is expressed to better advantage if the irregular cases are 

 not included. Removing the last column from the former table 

 v^e get the relations expressed in the following one: 



An examination of this table shows that 57 per cent of the "nor- 

 mal" cases have the functional operculum on the right and the 

 rudimentary on the left, while 43 per cent have the opposite 

 arrangement. The striking agreement in the three sets of figures, 



