Co7vpensatory Regulation. 



25 



in short blunt points. The two opercula are equal in size and the 

 stalks which bear them retain the pinnules and other branchial 

 characters (Fig. 6c). When the animal has withdrawn into its 

 tube the branchiae are twisted in spiral form and the two opercula 

 are superimposed, the one upon the other. The more anterior 



Fig. 6. 

 A — End of branchiaof Protula. B — Club-shaped end of branchia of Salmacina (after de St. Joseph). 

 C — One of the two opercula of Filograna (after de St. Joseph). D — Tip of non-operculate branchia of 

 Apomatus ampuUifera ( X 17). E, F — Tip of rudimentary operculum (£) and functional operculum 

 (F) of same ( X 17). G — Distal portion of functional operculum of Serpula vermicularis ( X 19). 



one closes the tube after the manner of forms with but one oper- 

 culum. The more posterior operculum, therefore, serves as a 

 protection only in the cases where the barricade formed by the first 

 is not effective. As compared with Salmacina, to which it is 

 otherwise closely related, Filograna has two, more effective oper- 



