22 Charles Zeleny. 



right and left is thus fairly equal though there is a considerable 

 advantage in favor of those with the functional operculum on the 

 right side and the rudimentary on the left. Similarly in H. unci- 

 nata out of i6 specimens ten had the functional operculum on the 

 right side and six on the left, and in H. pectinata out of 41 speci- 

 mens 21 were right handed and 20 left handed. 



An examination of the internal structure of the branchiae and 

 opercula brings out a close agreement between the two in ana- 

 tomical details. Their morphological agreement has been espe- 

 cially emphasized by Orley and Meyer. Orley ('84) compares 

 the internal anatomy of the branchia and the functional operculum 

 in Serpula. He makes no mention of the rudimentary operculum. 

 According to him an operculum corresponds morphologically 

 with a branchial stalk, all the pinnules of which have been col- 

 lected at the end in one bundle. He describes the presence of an 

 axial blood vessel in both branchial and opercular stalks. In the 

 branchial stalk, however, he saw only one nerve trunk (the axial 

 one) while in the opercular stalk two lateral ones were shown. 

 Meyer ('88) showed the more complete similarity of the branchia 

 and operculum in Eupomatus uncinata ( = Hydroides uncinata), 

 while at the same time pointing out the incompleteness of Orley's 

 observations and the error in his mode of homology. He describes 

 three nerve trunks in both branchiae and operculum, although the 

 middle one is very small in the opercular stalk and does not reach 

 much more than halfway to the distal end. In the branchia the 

 two lateral ones likewise are very insignificant. Meyer points out 

 that this difference is probably due to the fact that the pinnules 

 and ciliated groove are innervated from the nTiddle nerve, so that 

 this has a greater development in the branchiae where pinnules 

 and ciliated groove are present than in the opercular stalk where 

 they are absent. The stalk of the operculum is thus made directly 

 homologous with a branchial axis lacking its pinnules. 



A study of the internal structure of the branchiae and opercula 

 of Hydroides dianthus brings out points which are in entire agree- 

 ment with the conclusions of Meyer and which emphasize the 

 close similarity of the branchiae and opercula. 



An interesting characteristic is further made out in the func- 

 tional operculum. There is a difference between the cells near 

 the basal region and those in the middle and terminal regions of 

 the stalk. Near the base of the stalk in the region below the 



