Compensatory Regulation. 



27 



dorsal to the first dorsal branchiae or to the interval between the 

 first and second dorsal ones. Serpula differs from Hydroides in 

 the entire absence of the secondary group of processes in the oper- 

 culum (Fig. 6g). Crucigera has only four secondary processes 

 and these are arranged in the form of a cross (de St. Joseph, '94). 



Fig. 7. • 

 Spirorhis Pagenstecheri. Ventral (slightly anterior) view showing branchiae and operculum with its 

 brood chamber containing embryos ( X 40). 



f. Group VI . One Operculwn. No Rudimentary Operculum. 

 The members of this group have only one operculum. There is 

 no rudimentary operculum. Examples are Spirorhis, Pileolaria, 

 Ditrupa, Filogranula (.?), Pomatoceros, Vermilia. 



This group may be further subdivided according as the oper- 

 culum has a position in the line with the branchiae (Ditrupa, 



