380 



p. Kramp 



iransformation of the upper part of this, just an Broch has described. 

 — Bui to obtain complete proof,' we must follow the ontogenetic 

 course of development. 5/1 



The young hvdrotheca (PI. XXIV, fig. 8) ends in an aperture 

 with entire rim which is closed by a thin roof, already distinct from 

 the lateral wall; there is as yet no indication of the operculum. At 

 later stages we find the above-described lines; no folding can yet 

 be seen and the roof is still unbroken (PI. XXIV, fig. 4). When the 

 hydranth withdraws from the wall to form its tentacles, the oper- 

 culum accompanies it, presumably because the latter is still in close 

 connection with the hydranth, and will thus begin to fold up; as a 

 result the thin roof will be ruptured, perhaps falling otT at once or 

 perhaps remaining for a time as a crumpled mass, just as in Toicho- 

 poma (see p. 375); if the roof is not thrown olï beforehand, it will 

 naturally fall off when the completely formed hydranth extends. It 

 may happen that the operculum, when withdrawn as above-mentioned, 

 is pulled right into the hydrotheca; in such cases a distinct edge 

 appears as the lower boundary of the operculum; this edge is thus 

 a secondary phenomenon лvithout fundamental importance. 



Calijcella. — The operculum in Calycella is separated from the 

 hydrothecal wall by a sharp edge, which projects somewhat as a 

 rule, and which is seen to be thickened when the operculum is 

 open. If we open the operculum, we find that it is a cylindrical 

 membrane with a number of parallel, vertical lines; the rim of the 

 membrane is scalloped in correspondence with these lines; the 

 tongues are angular (PI. XXIV, figs. 12, 13) or rounded (PI. XXIV, 

 fig. 11); the depth of the indentations between may vary somewhat. 



How is it then, that this operculum appears as if it consisted 

 of triangular facets with intermediate parts which must fold in when 

 closed? If we look at the closed operculum from above (Plate XXIV, 

 fig. 14), we see a number of radiating lines meeting almost in the 

 centre of the operculum, and other lines which run out from the 

 same points in the periphery but which lie obliquely, all to the 

 same side of the radial lines; the intermediate, narrow parts appear 

 darker because there are here 3 layers of chitin. It is obvious that 

 the radial lines have been taken to be the outer, the oblique lines 

 the inner contours of the hypothetical, thin parts between the 

 triangular facets. A section perpendicular to the middle of an edge 

 will thus give the picture sketched in text-fig. 7 a. In reality the 

 oblique lines are external and they are the same lines we see on 

 the open operculum, whilst the radial lines are the contours of the 

 inner lying parts of the segments of the operculum; there is no 

 sharply marked fold in the chitin corresponding to this contour-line. 



