CONTROL OF ASYMMETRY IN HYDROIDES 941 
July 29. 
Operation. The distal half of the left second or future opercu- 
lar branchia and the tip of the left third branchia were removed 
as shown in fig. 2. 
August 3. 
Left side. From the branchia next to the dorsal one a functional 
operculum of the Serpula type has developed. The stalk has al- 
ready lost its pinnules (fig. 5). 
Right side. Rudimentary operculum (fig. 5). 
August 7. 
Left side. Rudimentary operculum. 
Right side. Functional operculum of Hydroides type, one- 
half developed. 
August 14. 
Left side. Rudimentary operculum (fig. 7, D). 
Right side. Functional operculum of Hydroides type. Not 
fully developed as yet (fig. 7, Z). 
The experiments thus answer the three questions proposed at 
the beginning of the paper. 
1. Is there a reversal of opercula as a final result of removal of 
the first functional operculum before any rudimentary opercular 
structure has been formed? The experiment shows that there 
is no reversal of opercula. A new functional operculum develops 
in place of the removed one, and on the opposite side of the body 
the final result is a rudimentary operculum like the normal one. 
Before this final condition there is, however, the interposition, 
as a result of the operation, of a new kind of opercular modifi- 
cation, namely, a rudimentary opercular enlargement at the end 
of abranchia. The enlargement is formed from cells which do not 
normally produce an operculum. ‘The formation of the struc- 
ture is directly stimulated by the operation. It has, however, 
no permanent result, the operculum thus formed in no way pre- 
venting the dropping off of the branchia. 
2. Is the removal of the first functional operculum followed by 
the regeneration of a structure of the same type as the one removed 
or does the later or adult type develop at once? It is shown that 
the new structure is like the one removed. It does not skip to 
