932 CHARLES ZELENY 
ASYMMETRY IN YOUNG INDIVIDUALS 
The adult asymmetry of Hydroides is preceded by a symmet- 
rical stage. With the development of a functional operculum 
the first asymmetrical phase is assumed, followed in turn by a 
normal reversal in position and a change in structure to the adult 
type. One or more additional reversals may then take place. 
It was shown that these reversals occur in nature and furthermore 
it is probable that they may come periodically without being pre- 
ceded by any definite injury to the functional operculum other 
than the wear of ordinary use. 
The free-swimming larva of Hydroides, after attachment to a 
solid object, secretes a tube around its body and develops bran- 
chiae on its head. There is a definite stage in which.the branchiae 
are all alike and symmetrically arranged with respect to the 
median line (fig. 2). Additions to these branchiae take place at 
the lower edge of each lateral group. At a stage slightly older 
than the one shown in fig. 2 the young serpulid closely resembles 
in its branchial characters an adult of the genus Protula (Fritz 
Miiller ’64). There is no trace of an opercular modification 1a 
any of the branchiae. 
The branchia next to the dorsal one on the left side then be- 
gins to form a terminal cup with a single row of serrations (figs. 3 
and 4). The branchial filaments are still present and the respir- 
atory function is evideatly still retained along with the new tube- 
plug function. The opercular branchia at this stage may be 
compared with the functional operculum of adults of the genus 
Apomatus, in which also the opercular enlargement is on a stalk 
bearing branchial filaments though the enlargement itself is dif- 
ferent in structure. Apomatus has in addition a rudimentary 
operculum of the same type as the functional, 7.e., borne on the 
end of a branchia which retains its respiratory filaments (fig. 6, 
A, B,C, D,). It is important to note that the operculum in Hy- 
droides first appears on the left side of the body while in adults 
it is sometimes on the right and sometimes on the left. 
During the following period the branchial filaments disappear 
from the opercular stalk and at the same time the corresponding 
