76 Charles Zeleny. 



The instances so far given include those cases only which involve 

 the regeneration of the opercula alone. When the anterior por- 

 tion of the body is removed by a transverse cut the regeneration 

 at the anterior end of the posterior piece follows a similar series as 

 regards the development of the opercula, but the manner of regen- 

 eration of the branchial circlets shows a striking similarity to that 

 of the first appearance of these organs in ontogeny. In its first 

 stages the circlet consists of a group of four buds, one of which from 

 the first shows its opercular character. The number of these 

 buds recalls very strikingly the number in the ontogeny. The 

 operculum here also, however, shows no trace of the branchial 

 characters so evident in its ontogeny. 



The data furnished by the opercula of the Serpulids, therefore, 

 give a fairly close agreement between the ontogenetic stages and 

 the probable phylogenetic ones as determined by the usual criteria. 

 The regeneratory development, however, follows a course which 

 may be modified bv the character of the operation that leads to 

 the regeneration. 



4. General Discussion of the Facts of Compensatory Regulation in 

 the Opercula of Serpulids. 



The data of both ontogeny and regeneration show that when the 

 opercula have an equal start in development they develop as 

 equal organs. When, however, one has an advantage over the 

 other as regards the time of starting it exerts a retarding influence 

 upon the other and holds the latter in a rudimentary condition. 

 The rudimentary operculum very evidently is either continually 

 held in check by a stimulus from the opposite fully developed 

 operculum or else there is no such continual retarding stimulus, 

 and the removal of the functional produces the positive stimulus 

 for further development, or both these conditions may hold. The 

 manner in which the presence of the large organ on one side acts 

 upon the smaller organ is, of course, not known, but there are 

 various arguments in favor of the view that the control may be 

 a nervous one. The experiments of Wilson ('03) on the nervous 

 control of the process of reversal of the chelae in Alpheus, my obser- 

 vations on the regeneration of the branchial circlets and tail knobs 

 in Serpulids in connection with the cut ends of the here widely 



