74 Charles Zeleny. 



toceros triquetroides the operculum is always on the left side 

 though the tube is irregularly coiled. 



The functional operculum in the young Hydroides always 

 appears on the left side though later in ontogeny the correspond- 

 ing branchia of the other side is modified into a rudimentary 

 operculum which possesses the power of developing into a func- 

 tional one under certain conditions usually connected with injury 

 to the operculum of the opposite side. The tube is irregularly 

 coiled. 



The constancy of position of the operculum in Pomatoceros 

 and of the functional one in the larval Hydroides, notwithstand- 

 ing the irregularity of the coils in the tubes, indicates that these 

 forms may be descended from ancestors with a definitely coiled 

 tube with which, further, the definite position of the operculum 

 was associated. The fruitlessness of such speculations is, how- 

 ever, very evident, especially if an attempt be made to apply the 

 same reasoning to the case of Vermilia multivaricosa, in which 

 the single operculum may be either on the right or on the left side. 

 The coils are here also irregular. Would it be necessary to 

 assume a descent from a definitely {i. e., not irregularly) coiled 

 species which produced both dextral and sinistral, and, therefore, 

 right and left operculate individuals in a manner corresponding 

 with that of some of the species of snails.^ Speculation from 

 this point of view seems to be of little value. 



In connection with this discussion it may be well to state that 

 Vermilia multivaricosa evidently comes under the head of the 

 cases which Conklin ('03) seeks to explain on the basis of a reversal 

 in the polarity of the egg, as there is no mechanism for the reversal 

 of the opercula in post-embryonic development. The two condi- 

 tions, right and left, in the present case are of practically equal 

 occurrence. 



3. Comparison of Regeneratory, Ontogenetic and Probable 

 Phylogenetic Development, 



As the data obtained from ontogeny have been used in the 

 determination of the probable phylogeny the probability of the 

 course of the phylogeny as given in the present discussion is 

 weakened by the removal of these data though such a removal 

 is made necessary by the character of the comparison. Neverthe- 



