Compensatory Regulation. 



45 



one. Branchia II — i has eight pinnules. The character of 

 Branchia III was not made out. 



The operculum was seen for the first time six days later at the 

 stage shown in Fig. 15. Fig. 14, taken five days later still (34 days 

 old), shozus an earlier stage of the operculum. Here it appears as a 

 rounded kf^ob on the 

 end of Branchia L-II . 

 This branchia has at 

 this time nine branch- 

 lets counting the one 

 (II — i) which has as- 

 sumed the character 

 of a main branch. 

 The eight represented 

 as pinnules are II — 2 

 to II — 9. The knob 

 at the end of the 

 branchia is conical in 

 form with the base of 

 the cone free and the 

 apex attached to the 

 stalk. The corres- 

 ponding branchia on 

 the right side (R-II) 

 has likewise at this 

 stage nine branchlets 

 counting the independ- 

 ent one (II — i) or eight 

 dependent ones (pin- 

 nules) II — 2 to II — 9 

 without this, but there 

 IS no modification at 

 the tip of the main stalk 

 as occurs on the left 

 side. 



Fig. 13. 

 Hydroides dianthus. Age, 23 days. Dorsal view (X75). 

 Original third pair is not shown. First pair shows beginnings 

 of secondary branches. 



In Fig. 15 the opercular character of the knob on L-II is very 

 evident. The cup of the operculum has a notched edge with 

 eleven serrations and resembles in its character the Serpula type 

 and not the Hydroides type. The eight dependent (II — 2 to 

 II — 9) and one independent branch are very prominently developed 



