Compensatory Regulation. 



47 



increasing in size and now has 12 pinnules plus a thirteenth bud on 

 each side Branchia III is also branched but the character of its 

 pinnules is not given in the figure in order that complication may 

 be avoided. This pair of branchiae is directed away from the 

 observer toward the ventral side of the animal. At all these stages 

 the Branchiae L-II 

 and R-II both retain 

 their flexible and 

 branchia-like char- 

 acter in all respects 

 except for the ter- 

 minal enlargement 

 in L-II. 



In Fig. 16 the pin- 

 nules of L-II have 

 disappeared, leaving 

 a cup-like opercu- 

 lum with a serrated 

 edge on the end of a 

 long, slender, flexi- 

 ble but bare stalk. 

 The corresponding 

 branchia on the 

 other side (R-II) has 

 dropped off, leaving 

 only a small round 

 knob at the place 



of its former attach- p.^^ 



ment. The other ^ a a a- .x. a ^ a cu u u j , 



, Hydroides dianthus. \gt, 28 days. Shows the three most dorsal 



Drancniae, i, ll I pairs of branchia. The ventral pair directed awav from observer is 



and III, all are keep- pot shown. The opercular knob of the left side is notched and its 



mg on with their ^^^^^ ^^^ retains the respiratory pinnules. The next to the dorsal 



branchial develoo- '"'^°*^^*^ °^ ^^^ "S^"^ ^^^^ ^^^ ^'S^^t pinnules but differs from other 



^1 • 11 branchia, except the opercular one, in the absence of new pinnule 



ment by contmually ^^^^ ^ r . v 



adding new pinnules 



to the old ones. We thus have three pairs of branchiae at this 

 stage with a functional operculum of the Serpula tvpe on the left 

 side and a rudimentary operculum on the right side. Xo vouno- 

 Serpulids were observed that showed an exception to this order of 

 appearance of the opercula. Numerous observations were made 



