52 Charles Zeleny. 



dependent condition increase in strength, develop secondary 

 branches of their own and soon take their place as independent 

 branchiae coequal with the three primary pairs. After reaching 

 its limit of growth as a branchia the next to the dorsalmost 

 branchia on the left side starts a new differentiation at its end, 

 developing a knob which rapidly increases in size and soon 

 assumes the shape of an inverted cone. Along the edge of the 

 upturned base notches appear so that the whole knob has the 

 general character of the opercular cup of members of the genus 

 Serpula. All this time, however, the stalk has retained its eight 

 branchial filaments and the corresponding branchia on the other 

 side has remained unchanged. This stage corresponds in general 

 with the adult of Filograna or rather with Apomatus except that 

 only one operculum is present. The branchial filaments of the 

 stalk, however, soon disappear. Whether they drop off or are 

 resorbed was not made out but the former supposition is the more 

 probable one, as they were still very long a short time before they 

 had entirely disappeared; or, in other words, no intermediate stages 

 of resorption were seen. Almost coincidentally with the disap- 

 pearance of these pinnules the next to the dorsal branchia of the 

 right side drops off, the region of the break being near the base. 

 From this broken stump a bud develops which in a few days has 

 reached its limit of development for the time being. This bud, 

 which reynams as the primary rudimentary operculum for several 

 months, is a regenerated structure, a true case of physiological 

 heteromorphosis. Furthermore, it is restricted in its development 

 by some forces acting from without its own substance. At this 

 stage the opercula remain for a considerable time (several months) 

 with no further change, although at the same time the animal is 

 increasing in size, is building up its tube and new branchiae are 

 being added on the ventral edge of each of the branchial ridges. 

 In its essential characters this stage is equivalent to that of adult 

 members of the genus Serpula with the functional operculum on 

 the left side. 



After this long period of no opercular change the primary func- 

 tional operculum drops off, the stalk breaking near its base. 

 Immediately the primary rudimentary operculum on the right 

 side, no longer restricted by outside forces, starts its further devel- 

 opment and becomes a functional opercular organ. However, it 

 does not develop into an operculum of the siinple type like the pri- 



