152 



HYDROIDA II 



Here, as elsewhere, when drawing the limits of a genus, we must seek to emancipate ourselves 

 from the interesting biological features of the gonophores, and have recourse to Levinsen's funda- 

 mental point of view; the study of the nutritive polyps and the colonies themselves. 



We find then, that the family falls into two great groups, according to the diaphragm of the 

 polyp, the arrangement being further accentuated by the two fundamentally different colony forma- 

 tions, the stolonial and the sympodial. Levin sen (1893) believed to have discovered that the dia- 

 phragma of the one group or genus Campamdaria was a double formation, consisting of a cen- 

 tral, thin membranous diaphragm, proceeding from a circular inner thickening of the wall, the 

 boundary between the main cavity of the hydrotheca and the basal chamber. A thorough investiga- 

 tion of the point has convinced me (1909 p. 183) that Levinsen's conclusion must be due to his not 

 having employed microtome sections; for microtome series reveal the fact that the "diaphragm" in 

 the Campamdaria group is produced by the high power of resistance against dissolvent influences 

 which characterises the basal part of the supporting lamella, and that a true diaphragm, i. e. a chitin- 

 ose, membranous bottom under the basal ectoderm, is altogether lacking. 



Nutting (1915 p. 9) has arrived at a different result. He supports his view upon an unpub- 

 lished manuscript of his pupil, J. H. Paarmann, and on original preparations by the same hand. It 

 is on Paarmann's original drawings also, that Nutting's text figures 2444 are based. These figures 

 appear to have been sketched from optical sections, not from microtome series, and present altogether 

 the impression of being not particularly reliable. Nor is this impression removed by the following 

 passage quoted by Nutting from Paarmann's manuscript: "The simple diaphragm can with diffi- 

 culty be seen in optical section of the hydrotheca, while the complex diaphragm is plainly distinguish- 

 able without sectioning". Now the fact is, that the thickening of the wall in Campamdaria is as a 

 rule fairly easily visible, while the true diaphragm here, in such North-European species as I have 

 been able to investigate, does not even show up in microtome section. Paarmann has figured the 

 "complex" diaphragm also in our European species Campanularia verticillata (Linne). He gives, as a 

 matter of fact, two figures of this, and the two are not very much alike (Nutting 1915, figs. 38 

 and 43); I have not, however, succeeded in finding the membranous part of the diaphragm in Euro- 

 pean specimens of the species. Paarmann's drawing also reveals a peculiarity in Laome dea flexuosa 

 Hincks (1. c. text figs. 9 and 26), the free margin of the diaphragm being here double, a phenomenon 

 which is not discernible in Kiihn's illustration (reproduced in Nutting's text fig. 47) and is similarly 

 lacking in my preparations. Details of this sort are apt to cast some doubt upon the value of Paar- 

 mann's drawings. We should note, however, that all the species in which Nutting, following 

 Paarmann, finds a "simple" diaphragm, have sympodial colonies, while among species with "complex" 

 diaphragm only two species are cited: Obelia geniculata (Linne) and Obelia flabellata Hincks, which 

 have no stolonial colonies. From this is might be supposed that the two species should be regarded 

 as types of a distinct genus, but this is not the case. Even in Paarmann's drawing (Nuttingigis, 

 text fig. 41) the diaphragm of Obelia flabellata is simple; somewhat thicker, it is true, than in most 

 Laomedea species, but by no means resembling the broad wall thickening in Campanularia. This, 

 together with the sympodial growth of the colony, places the species undoubtedly in the Laomedea 

 group. With regard to Obelia geniculata, it might be a somewhat different matter. This species 



