Compensatory Regulation. 



51 



plus a very small bud on the left side and six branchiae plus a large 

 bud on the right. The functional operculum has a basal funnel- 

 shaped cup with a serrated edge. From the upper flat end of this 

 cup there projects a new secondary cup, the individual serrations 

 of which reach nearly to the base and are strongly toothed. 



B 



r\ 



c 



Fig. i5 



Hydroides pectinata. Age, 181 days or 6 months. A — Ventral (somewhat inclined) view, showing 

 position of secondary functional and rudimentary opercula. Pinnules of branchiae are not shown in 

 the figure (X 16). 5— Secondary functional operculum (adult Hydroides type with two rows of pro- 

 cesses) ( X 32). C — ^Rudimentary operculum ( X 32). 



d. Summary of Data and Discussion. The above results con- 

 cerning the ontogenetic development of the opercula may be sum- 

 marized as follows : 



At the stage with four pairs of branchiae the next to the dorsal 

 one on each side stops its development w^hen it has eight long 

 slender pinnules. The two branchiae of the third pair, counting 

 from the dorsal side, arise originally as branches which resemble 

 in all respects the ordinary pinnules, but instead of retaining their 



