Compensatory Regulation. 3 1 



processes and also has two lateral wings like those of Pomatoceros 

 elaphus but not as well developed as the latter. E. Meyer ('88) 

 concludes that these opercula have been formed by the union of 

 two lateral ones, but the evidence as regards this point is by no 

 means conclusive, since we have species where similar opercula are 

 evidently lateral in position (P. triquetroides and V. multivaricosa, 

 for example). 



g. Siiininary. The principal modifications of the opercula 

 throughout the family of Serpulids have now been passed over 

 briefly and the general characters may be summarized. In the 

 first group are the forms with no opercular modification (Protula, 

 Protis). In the second each branchia has a small club-shaped 

 enlargement, the combination of the enlarged ends no doubt 

 making a more or less effective barricade against invaders when 

 the animal has retreated into its tube (Salmacina). In the third 

 group (Filograna) the modification is confined to the most dorsal 

 branchia on each side. All the others lack opercular differentia- 

 tions. The two dorsal ones mentioned also retain their branchial 

 characters, but in addition each has an operculum of sufficient 

 size to close up the opening of the tube. In the fourth group 

 (Apomatus, Josephella) there is one functional and one rudi- 

 mentary operculum, one on the end of each of the two next to the 

 dorsal branchiae. The stalks supporting these opercula retain 

 their branchial pinnules and other branchial characters. In the 

 fifth group (Serpula, Crucigera, Hydroides) there are likewise 

 one functional and one rudimentary operculum, the distribution 

 between right-handed and left-handed forms being fairly equal 

 in adults. The opercula, however, do not possess branchial 

 pinnules though their internal anatomy and position indicate 

 branchial characters. The rudimentary operculum is not situ- 

 ated on the end of a long stalk but is a small bud-shaped organ 

 corresponding in position with the functional operculum. Judg- 

 ing by their position the opercula seem to have moved dow^n from 

 the interval between the most dorsal and the next to the dorsal 

 branchiae on each side. Finally in the sixth group there is only 

 one operculum and this retains but little of its branchial character. 

 In some of the group, however (Ditrupa, Spirorbis, Pileolaria), 

 it retains its position in the branchial circlet. In some cases it 

 may be used as a brood pouch (Spirorbis, Pileolaria). In other 

 forms (Pomatoceros, Vermilia) it is a considerable distance below 



