Compensatory Regulation. 55 



H. pectinata and Serpula vermicularis, then a member of Group 

 IV, Apomatus ampullifera, and finally the members of Group 

 VI, Ditrupa subulata, Spirorbis Pagenstecheri, Pomatoceros tri- 

 quetroides and Vermilia multivaricosa. The adult condition of 

 the opercula which has already been described in detail in the 

 anatomical portion of the paper will be again briefly noted, as it 

 must serve as the basis of our experiments. The experiments 

 will then be described in turn, and finally the results will be 

 discussed. 



b. Unoperated Coyidition of the Opercula in Hydroides Dianthus. 

 The character of the adult opercula has already been given on 

 p. 21 f[l. and is also shown in Fig. 5E, f. A repetition of these 

 data is therefore unnecessary. 



In all the experiments about to be described the animal was first 

 removed from its tube and placed in a dish of sea-water, the desired 

 operation was performed under a dissecting microscope and the 

 animal was kept in its individual dish either with running or 

 standing water, in the latter case the water being changed once or 

 twice a day as required. The running water was not found as 

 favorable as the standing because of the collection of a fine deposit 

 on the animals notwithstanding the greatest care exercised. The 

 dishes with standing water were found very favorable if provided 

 with a glass cover to keep out the dust. The observations were 

 in most cases made on the living animals. 



c. Operations on Functional Operculum. The results may 

 best be arranged around a description of the effect of 

 a cross cut through the stalk three-fourths of the distance 

 from the base to the beginning of the terminal expansion. 

 The stump of the functional stalk remains attached to the animal 

 for two or three days as a rule or even longer in some cases. It 

 then breaks off from the body, separating by a clean division at the 

 basal suture or "breaking joint" described above (pp. 21-23). On 

 the distal end of the small stump still remaining attached to the 

 animal a small bud now appears and gradually increases in size 

 until it reaches the dimensions and character of the former rudi- 

 mentary operculum of the opposite side. At this point it stops and 

 proceeds no further. In order to understand the result it is neces- 

 sary to look away from the immediate vicinity of the operated organ 

 and to note the change going on in a corresponding position on the 

 other side of the animal. Even before the attached stump of the 



