T^o HYDROIDA II 



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

 from the interesting biological features of the gonophores, and have recourse to Levin sen'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) beHeved to have discovered that the dia- 

 phragma of the one group or genus — Caiiifianiilaria — 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 cavit)- 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 Campanularia 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 24—44 are based. These figures 

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

 the inipres.sion of being not particularly reliable. Nor is this impression removed b\ 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 plainh' distinguish- 

 able without sectioning". Now the fact is, that the thickening of the wall in Campanularia 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 vcrticillata (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 Lao)nedca flcxuosa 

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

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

 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 sj^mpodial colonies, while among species with "complex" 

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

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

 as types of a distinct genu.s, Ijul this is not the case. Even in Paarmann's drawing (N uttin g 1915, 

 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 s\-mpodial growth of the colony, ])laces the species undoubtedl\- in the Laomedea 

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



