928 CHARLES ZELENY 
resemble the one removed or does it develop directly as an oper- 
culum of the adult type? 
3. Further, since the first operculum is a modification of a 
branchia and therefore originally has a respiratory function, is 
its removal followed by regeneration, first as a branchia which 
only later develops the opercular modification, or is the opercular 
modification regenerated directly? 
For a description of the opercula in adults and for experiments 
on reversal, reference is made to two former papers (Zeleny ’02, 
705). The development of the opercula in the young is treated 
in the second of these (pp. 38 to 54). A brief outline of the neces- 
sary points must suffice here. 
ASYMMETRY IN ADULTS 
In the adult Hydroides dianthus there is a large functional 
operculum or tube plug on one side of the body, either right or 
left, and a small rudimentary operculum on the other side (fig. 
1). The functional operculum (fig. |, /”) has a stout stalk end- 
ing in a hard chitinous enlargement consisting of a serrated cup, 
from the centre of which rises a circlet of curved spine-like pro- 
jections. The genus Hydroides is characterized by the presence 
of these two separate circles of serrations or projections in its 
functional operculum. ‘The rudimentary operculum (fig. 1, /) 
is a small bud-like structure corresponding in position exactly 
with the functional operculum of the opposite side of the body. 
Near the base of each opercular stalk there is a well defined line, 
the breaking line or breaking joint. 
When the cup of the functional operculum is removed, the 
rudimeatary operculum of the opposite side develops into a func- 
tional operculum similar to the one which had been injured. The 
stalk of the inj:1red operculum meanwhile drops off at its break- 
ing joint and a rudimeutary operculum develops in its place. 
There is thus, as a final result of the operation, a reversal in posi- 
tion of the two opercula. This reversal may be repeated several 
times. 
