Report on the Hj^droids. 



381 



л 



Text-fig. 7. 



nor do we find any corresponding oblique line on the segments in 

 the unfolded operculum; if we look at the closed operculum obliquely, 

 so as to see along one of the edges, we 

 find that this projects like a keel lying 

 somewhat to the one side, and it лу111 

 then be understood, how a section per- 

 pendicular to the middle of an edge gives 

 the picture represented in text-fig. 7 b. 



How then has this operculum devel- 

 oped. — Probably the fact that the lower 

 part of the opercular membrane is cylin- 

 drical, has led Broch to the view, that 

 the operculum here as in Ciispidella is 

 formed of the uppermost })art of the 

 hydrothecal wall ; this A'iew is not correct. 

 РЪг Calijcella Levinsen is right in thinking 

 that the operculum is formed of the ori- 

 ginal roof of the hydrotheca. Plate XXIV, 



figs. 7 — 10 show some young hydrothecae, in the roof of which the 

 radiating lines are visible, as described by Levinsen. Usually the 

 roof is arched into the hydrotheca (figs. 7, 8); fig. 9 shows a hydro- 

 theca in which the roof has been artificially squeezed out; fig. 10 

 shows a specimen from Iceland. When the hydranth is about to 

 form its tentacles it retreats from the wall (just begun in fig. 7, 

 further advanced in fig. 8) and from the roof, which thereafter does 

 not undergo any change until the completed hydranth extends; then 

 the central part of the roof is thrown oil. The opercular membrane 

 remaining is narrower above than below and will naturally split 

 along the radiating lines. If the roof is high, dome-like and in its 

 lower part cylindrical (figs. 7 — 9), the portion thrown off will be 

 comparatively large, and the splits in the opercular membrane will 

 be but short (figs. 11 — 13); if the roof is low (fig. 10) only a small 

 part is thrown olT and the splits in the opercular membrane will go 

 further down. All the specimens examined from the Danmark Expe- 

 dition have a high operculum. In all cases the "facets" of the oper- 

 culum will become angular. The folding and closing of the operculum 

 takes place in the above-described manner; and then all the corners 

 of the facets lie uncovered and exposed to rubbing; this causes the 

 rounded tongues we find in the older hydrothecae, which are always 

 frayed at the rim (fig. 11). 



I have seen all the developmental stages necessary to give this 

 picture and I maintain : 



In Cuspidella the operculum is formed of the uppermost 



