930 CHARLES ZELENY 
removed, the cutting chela changes into a snapping chela and in 
place of the former snapping chela a cutting one is developed. 
In Hydroides the presence of the functional operculum in some 
way inhibits the growth of the rudimentary operculum into a 
functional one. Likewise in Alpheus the presence of the snap- 
ping chela inhibits the change of the cutting chela into a snapping 
chela. It is evident that the rudimentary operculum in the one 
case and the cutting chela in the other need only the proper stim- 
ulus or removal of an inhibition to develop at once into organs like 
their mates. These smaller organs are therfore in one sense 
merely stages in the development of the others. 
Various suggestions as to the explanation of this inhibition have 
been made. In Hydroides the injury to the large operculum pro- 
duces activity on both sides of the body, bringing about the 
immediate growth of the rudimentary into a functional operculum 
and astart in the same direction to form a rudimentary operculum 
on the side of the injury. The functional operculum is reached 
only on the side with the earlier start, the presence of the one 
functional operculum restraining the possible development of 
another functional one. This view is supported by the result 
obtained when the head of Hydroides is removed. In this case, 
as also in a part of the cases in which the two opercula are 
removed without injury to the body proper, a functional opercu- 
lum is developed on each side of the head though the size of each 
is usually reduced. In such a case the two opercula get an equal 
start and neither one is able fully to restrict the development of 
the other. 
The cases of similar chelae in the adult Alpheus and Homarus 
may be explained in a similar way by coincident development. 
Under the ordinary conditions of removal of both chelae the ad- 
vantage of greater blood-supply and probably other features lies 
oa the side of the stouter snapping or crushing chela. Therefore 
reversal does not occur in such cases. When, however, there is 
a secondary advantage of just the proper strength occurring to 
the side of the former smaller chela two equal chelae may develop. 
The probability of this explanation is strengthened by the fact 
that in those cases in which two equal snapping or crushing chelae 
