24 Charles Zeleny. 



The increase in strength of the supporting axis is a necessary con- 

 comitant of the other changes. 



The rudimentary operculum cannot be compared directly with 

 a branchia because of its bud-Hke appearance and embryonic 

 tissues. In position it, however, corresponds perfectly with the 

 functional one and therefore with the branchiae as well. 



2. Other Genera of Serpulids. 



An examination of the different groups of Serpulids brings out 

 the fact that we have almost all gradations between forms with no 

 opercular modification of the branchiae and forms with the single 

 operculum possessing scarcely any trace of a branchial character. 



a. Group I. No Opercular Differentiation. Examples of 

 Serpulids with no opercular modification of the branchiae are 

 Protula (Risso) and Protis (Ehlers). Each branchia possesses 

 respiratory pinnules and tapers to a point at its distal end. The 

 branchiae resemble one another throughout both right and left 

 circlets. The members of this group are able to retreat for a long 

 distance back into the tube, in this respect resembling the Sabellids 

 which also have no opercula. (See Fig. 6a.) 



b. Group II. Each Branchia with a Terminal Enlargement. 

 In Salmacina Dysteri Huxl. there are eight branchiae, four on each 

 side and each has a terminal club-shaped enlargement. The 

 branchial stalk or axis has from fifteen to twenty pairs of ciliated 

 pinnules. The two rows of pinnules are bordered on the outside by 

 enlarged mucous cells which near the distal end spread out along 

 the sides of the club. This enlarged region bears no pinnules. 

 The eight branchiae are similar in their characters. It is evident 

 that when the animal retreats into its tube these enlarged ends 

 must collectively serve as a stopper for the opening and thus barri- 

 cade the end more effectively than those of Protula which bear no 

 such enlargements (Fig. 6b). ^ 



c. Group III. Two Equal Opercula, Right and Left, on Ends 

 of Branchice. Branchial Pinnules Present. Filograna implexa 

 resembles Salmacina in having eight branchiae. The dorsal one 

 on each side is, however, terminated by a small, transparent, 

 chitinous, spoon-shaped structure obliquely attached to the side 

 of the tip of the axis of the branchia. The other branchiae end 



'de St. Joseph, however, seems unwilling to admit an opercular function for these structures. 



