Compensatory Regulation, 



35 



Likewise 41 specimens of Hydroides pectinata were examined 

 at Naples, and of these 21 or 51.2 per cent had the functional oper- 

 culum on the right side and the rudimentary on the left while 

 48.8 per cent had the reciprocal arrangement, a slight advantage 

 in favor of the right-handed ones. 



Table VI. Hydroides pectinata. Position of Opercula. 



Locality and Date 



F= Right F=Left 

 R=Left R= Right 



Total 



Naples, 1902/IX/27 

 1 903/1 1 1/3 1 



No. 

 Per cent 



21 

 51-2 



41 



In Serpula vermicularis I examined only one specimen and this 

 had the functional operculum on the left side and the rudimentary 

 on the right. 



Our general conclusion regarding the distribution of the oper- 

 cula in the -fifth group of Serpulids, those with the naked-stalked 

 functional and small bud-like rudimentary is that the right and 

 left-handed forms are nearly equal in number, but there is a slight 

 advantage in favor of the right-handed ones. 



(r.) In the sixth group there is only one operculum. 



In Ditrupa subulata only four specimens were examined as 

 regards this point. All four of these had the operculum on the 

 left side.* 



In the dextrally coiled Spirorbis Pagenstecheri eleven specimens 

 were examined and all had the operculum on the right side, while 

 in the sinstrally coiled Pileolaria the one specimen examined had 

 the operculum on the left. The observations of Caullery and 

 Mesnil ('96) show that in the species with dextrally coiled tubes 

 the operculum is always on the right side and in the sinstrally- 

 coiled ones always on the left side. 



Me St. Joseph C98) gives a description of Ditrupa arietina, O. F. Miiller (= D. subulata, Desh.) in 

 which he mentions the operculum as a structure of the left side in agreement with the present observa- 

 tions. Also the left side according to him has one less branchia than the right, ;. e., there are 1 1 branchiae 

 (plus the operculum) on the left and 12 on the right. 



