Cofiipi'7isatory Regulation. 65 



The effect of the cut upon the old organs is as follows: The 

 rudimentary operculum m several cases showed a slight develop- 

 ment though only one advanced to the stage with both rows of 

 opercular processes present. Usually the rudimentary operculum 

 remained unchanged or a slight development of the secondary 

 processes took place. No changes were observed under similar 

 circumstances in the functional operculum. 



The second group of experiments concerns the regeneration 

 from a posterior cut surface of a half (right or left) Serpulid. 

 Two of the cases of regeneration at the posterior cut surface after 

 longitudinal dorso-ventral section of the whole organism showed 

 very clearly the character of the new tail bud regenerating there. 

 After transverse section of the whole body in the abdominal 

 region the new tail bud is always very evidently double. In 

 the present experiments, however, where only half of the animal 

 was used the regenerating bud always showed a single tail knob. 

 As the two components of the ventral nerve-cord in Serpulids and 

 Sabellids are widely separated this singleness of structure may be 

 correlated with the presence of only one of these nerve cords at the 

 posterior end when the animal is cut in two longitudinally before 

 the cross cut is made. 



g. Progressive Changes m Opercida. Progressive changes in 

 the opercula of adult specimens were not observed. Several 

 groups of specimens were kept under observation for varying 

 periods of time but in no case was evidence of such a change 

 noted. It must, however, be stated that the periods were all rela- 

 tively short, not over a month at most. The indirect arguments 

 in favor of the occurrence of such changes as furnished by speci- 

 mens in nature with intermediate stages of reversal, etc., are given 

 elsewhere, (p. 33.) 



h. Experiments on Group V . (See p. 26 for definition.) A 

 series of experiments on H. uncinata was undertaken with the 

 object of determining the relative capacity for regeneration at 

 different levels in the body. The most posterior region showing 

 regeneration of branchial and opercular structures was an anterior 

 cut surface located between the next to the last (sixth) and the 

 last (seventh) thoracic segments. Several posterior pieces back of 

 this point lived for a sufficient length of time to allow of regenera- 

 tion if it were to occur at all, but all these healed up at the 

 cut surface and showed no regeneration of head structures. We 



