46 



Charles Zeleny. 



and evidently all of them retain their respiratory character. The 

 corresponding branchia (R-II) on the right side has branchlets 

 similar to those of the branchia (L-II) on the left side though it 

 shows no opercular modifications. It is similar to the latter and 

 different from all the other branchia? in one essential respect. 

 While I, II — I and III show buds of new developing pinnules its 

 pinnules (/. e., the eight dependent ones II — 2 to II — 9) are all 



Fig. 14. 

 Hydroides dianthus. Age, 34 days. The next to the dorsal branchia on each side. The left one 

 shows the beginning of the knob of the functional operculum. The right one later drops off and the 

 rudimentary operculum regenerates from the remaining stump. 



well grown, indicating, no doubt, that the organ has reached its 

 limit of development as a branchia. 



With the beginning of the opercular differentiation Branchia I 

 enters on a new period. It increases in size and new branchlets 

 (pmnules) sprout out in rapid succession, the first ones appearing 

 simultaneously. Thus in Fig. 15, I already has thirteen well 

 formed pinnules with several buds crowded into a zone at the 

 base of the terminal filament. Branchia II — i has likewise been 



