CONTROL OF ASYMMETRY IN HYDROIDES 935 
After a period of activity in this stage lasting several days the 
functional operculum drops off. The loss is accompanied by the 
development of the rudimentary operculum into a functional 
operculum not like the one that had been lost but more compli- 
cated in structure, with two distinct and qualitatively different 
X 
Fig. 5 Hydroidesdianthus. Dorsal view (X 38). Onthe left side is functional 
operculum of Serpula type with naked stalk and one row of serrations. On right 
side is rudimentary bud developed from the base of the cast-off second branchia 
of that side. 
rows of serrations. This operculum is similar to that of the adult 
Hydroides. In place of the operculum that had dropped off a 
rudimentary operculum is developed. The condition is now like 
that of the adult except that some adults have the functional oper- 
culum on the left and others on the right. It is therefore neces- 
