Compensatory Regulation. 69 



sensitive to the injury and nearly all the individuals died very 

 early. However, the beginning of the process of regeneration 

 was observed in a few cases. In one of these which had been 

 cut through the third thoracic segment there was, 23 days after 

 the operation, a distinct indication of the young branchial 

 circlets at the anterior end of the posterior piece in the form 

 of three branchial knobs on the left side and jour on the right. 

 (Fig. 23E, F, G.) Of the latter four the ventral one was very small 

 and the next to the dorsal one evidently larger than the others and 

 showing thus early its opercular character. The regenerating 

 tissue appears first as an undifferentiated mound. When dif- 

 ferentiation does occur it takes on the form of three or four knobs 

 in each mound, corresponding evidently with those of Hydroides 

 dianthus after a similar section and reminding one strongly of the 

 first branchial differentiation in the young of the latter species 

 where, as we have seen, each branchial circlet appears first as three 

 processes, one of which divides at its base, forming four in all. 

 The early differentiation of the opercular knob after a thoracic cut 

 in Apomatus as in Hydroides, however, brings in a point of dif- 

 ference as compared with the ontogenetic development. 



y. Experiments on Group VI . In four specimens of Ditrupa 

 subulata the functional stalk was cut in two just below the terminal 

 cup. The embryonic tissues at the base of the stalk increase in 

 bulk and bulge out, showing an oblique suture. The stalk drops 

 off a few days after the operation and a new operculum develops 

 from its stump. Evidently the increase in the embryonic tissues 

 serves as a mechanical stimulus for the dropping off of the old 

 stalk. See Fig. 24A, b. 



All the specimens of Spirorbis Pagenstecheri in which the 

 operculum was removed died. There is, however, evidence that 

 regeneration of the operculum takes place. A considerable 

 number of the specimens just removed from their tubes showed 

 stages of growth of the operculum from a small bud to a large 

 full-sized operculum. Whether the process is a direct physiolog- 

 ical one or is due to injury cannot of course be definitely stated. 

 A periodic replacement may be connected with a possible periodic 

 injury during the breaking out of the embryos from the brood 

 pouch, though evidence is also lacking as to the length of life of 

 the animals. 



Several experiments on the regeneration of the opercula in 



