20 AMERICAN HYDROIDS. 



Iii 1890 Marktanner-Turneretscher 1 used the opcrculuni in giving the diagnostic features of 

 the genera Xi'i'ti//(//vI/<t, Calyptothuiaria, .lA<//<y<///c, I>!J>/I<IK!<I, and Dynamena. In the present 

 work Cah/ptothuiaria is included in the genus S, ///<// ll<i and Monopoma in Ahict'tnnr'm. 



As before indieateil. it remained fur Levinsen to make a systematic and comprehensive 

 study of the operculnm in various groups, and this he has done with the painstaking care that is 

 characteristic of our Scandinavian fellow-zoologists. 



The following points are ((noted direct from his systematic discussion of the Sertularidse a 

 and translated by Mr. J. H. Paarmann, who studied the operculum in connection with a thesis 

 for the master's degree in the State University of Iowa: 



Our attention has before been called to tin- fart that an operculnm is found in all Sertularida', and that it, together 

 with the form of the margin of the hydrotheca, is the only character hy means of which we can draw a natural 

 boundary line between the Campanularidse and Sertularid;e. There is at least a certain relation between the 



form of the margin and the structure and position of the operculum. Thus a Si'rtiilnrillu thai lias lost its operculum 

 may he easily recognized by the three or four equally developed curves in the margin on which the operculum has 

 been attached; and in the genera in which the opercnlum is a single flap the attachment of the lost operculum cm the 

 inner or outer margin will be indieateil by a more or less deepened curve. We may deline the genus 



M-rliiliii-iii thus: Aperture of the hydrotlieca provided on the outer (abcauline) .side with a deep cmargination in 

 which the flap-formed operculum is attached; on the opposite (adcauline) side is found a thin portion ("kraven" = 

 collar) of similar form to the emargimttion mentioned above. It thus appears that on eaeli side of the margin is a 

 dentate or triangular projection, and that betx een these on the inner side is stretched a thin membrane. This 

 membranous part is, in general, overlooked by authors who describe or delineate the margin as bilabiate or two- 

 toothed. In a number of species this thinned portion of the wall (of the hydrotlieca), which we will call the "collar," 

 has been seen by Allman and Marktanner-Turneretsrher, who, however, have both misinterpreted it, regarding it as 

 a flap of the operculum, which, in combination with the real operculum, serves to close the aperture in the same 

 manner as the flap of an operculum in ,SVr//i(;r//i/. Not only the species which Marktanner-Turneretscher 



assigns to the genus lii/muiii-iui, but also the remaining species of the genus ,S' ( /// rrm, as we have defined it, have 

 such a collar, which, by this author, is incorrectly interpreted as a flap of an operculum. 



The above rather voluminous quotation from Levinsen has been given, because, in justice to 

 that writer, it is necessary that his position should be made as plain as possible in view of the 

 fact that the present writer is compelled to differ from Professor Levinsen and agree with 

 Allman and Marktanner-Turneretscher in his interpretation of the operculum of the type found 

 in Sci'tii/iirni. that is, the so-called " two-valved " operculum (see tigs. 38-53). 



The origin of this type in its developmental history should be understood in oi'der to 

 appreciate the points in discussion that will be presented later. If we examine a very young 

 hydrotlieca of Sertularia f>iu/iil<t, for instance, we will tind that its distal end is entirely covered 

 with a very thin homogeneous membrane, continuous over the entire surface. This is deposited, 

 like the hydrothecal walls with which it is continuous, by the ectoderm of the inclosed young 

 hydranth. There is nothing at this stage to show any distinction whatever between the hydro- 

 thecal walls and the operculum, the margin not yet having been differentiated. In preserved 

 specimens, however, the homogeneous membrane covering the hydrotlieca is apt to be wrinkled, 

 and these wrinkles may sometimes have the optical effect of structural characters. A little later, 

 but before the tentacles have become plainly differentiated, the hydrothecal margin appears by a 

 strengthening of the chitin, and can be traced as a fine, dark, sinuous line which marks the 

 outline of the two opposite teeth, which rapidly become more and more prominent. We have 

 now a distinct differentiation between the hydrothecal walls and that which is destined to become 

 the operculum, although there is yet no break whatever in the continuity between these two 

 structures, nor any opening at the distal end of the hydrotlieca. The future operculum is 

 shaped like the side walls of an "A" tent, the front and rear of the tent being closed by the two 

 opposite hydrothecal teeth. The two Haps of the tent are of unequal size, however, the abeauline 

 being considerably the larger. These two are nevertheless strictly homological structures, each 

 having originated in the chitinous pellicle that covers the distal end of the budding hydranth, 

 and each being adherent to the sides of the teeth and the portion of the hydrothecal margin 

 between them, the two uniting along the line that would be represented by the ridge pole of 

 the tent. 



1 Hydroiden des k. k. naturhistorischen Hofmuseums, 1S90, pp. 249-251. 

 "Meduser, Ctenophorer og Hydroider I'ra Gronlands Vestkyst, 1SH5, pp. 183-200. 



