CONTROL OF ASYMMETRY IN HYDROIDES 939 
the tip of a branchia is stimulated to develop. This condition, 
however, lasts only a short time. The branchia with its termi- 
nal enlargement breaks off as does the corresponding one in nor- 
mal development and a rudimentary operculum grows out in its 
place. While this is taking place the stalk of the regenerated 
functional operculum loses its filaments and the resultant condi- 
tion is like that of the Serpula stage of normal ontogeny (fig. 5). 
Outline of typical individual experiments 
Individual A. (No. 2778) 
July 1—1909. Egg fertilized. 
July 27. Condition of opercula (fig. 4). 
Left side. Second branchia has opercular enlargement. Res- 
piratory pinnules still present. 
Right side. No opercular modifications, Hach branchia ends 
in a long tapering filament. 
Operation. Distal half of second left (opercular) branchia 
and distal third of first left branchia removed (fig. 4, level A). 
Pub OL: 
Left side. Removed parts are regenerated. The new oper- 
culum resembles the removed one. 
Right side. The next to the dorsal (second) branchia has devel- 
oped a small ovoid enlargement at its end (fig. 7, B, C). The 
whole branchia now resembles a rudimentary operculum of the 
kind found in adults of the genus Apomatus (fig. 6, A, C). 
August 3. 
Left side. The new opercular stalk has lost its respiratory 
pinnules. The terminal cup retains a single circle of serrations 
(fig. 7, A). The structure resembles an adult operculum of the 
genus Serpula (fig. 5). 
Right side. The next to the dorsal (second) branchia with its 
small terminal enlargement has dropped off and a rudimentary 
operculum has developed in its place (fig. 5). 
August 7. 
Left side. Functional operculum of Serpula type (fig. 5). 
Right side. The former rudimentary operculum is going for- 
ward in its development to a functional operculum with two rows 
