HYDROIDA. BALE. 13 



is what takes place also in Thuiaria sertularioides, and the 

 recognition of which led Allrnan to remove the species from 

 Thuii.iria, in which genus he had at first placed it. In Thuiaria, 

 internodes bearing several hydrothecse are strictly normal, and 

 are developed continuously in the first instance, not formed by 

 the coalescence of a number of originally single-celled internodes 

 as in the varieties of 8. divaricat/i. Of course I do not dispute, 

 that these or other forms of Sertularella may also develop 

 continuous multiple-celled internodes, thus linking the two 

 genera, but I have never seen an undoubted SertnlareUa 

 in which the typical single-celled iuteruodes were not present in 

 some portions of the polypidom, nor in which the structure was 

 such as to forbid the assumption that all the interuodes had 

 originated in the same manner. Very commonly the stem of 

 Sertularella divaricata is divided by strongly marked nodes into 

 interuodes of three hydrothecffi, but between the component 

 hydrotheca? of these iuteruodes are often distinguishable less 

 pronounced constrictions, which evidently remain to indicate 

 where the original nodes have become almost obliterated ; while 

 in other varieties of the same species all remain equally distinct. 

 In Thuiaria, even in such a species as T. lata, not only are the 

 internodes without these vestigial joints, but it is usually obvious 

 that they have never been so divided, on account of the 

 hydrothecffl following each other so closely. Where nodes do 

 occur therefore they necessitate a gap between thehydrothecas on 

 either side of them, which is permanent, and the fact that the 

 hydrotheca? on an internode are so closely crowded and without 

 gaps between them is evidence that the internode has been 

 developed without interruption. 



It will of course be apparent from the foregoing remarks that 

 I am here adhering to the principle of classification which bases 

 the generic characters primarily on the grouping of the 

 hydrothecse, in short that I retain the genera precisely as they 

 were almost universally understood when my earlier papers were 

 written. The majority of observers, while retaining in the main 

 the same principle, have modified the genera in various directions 

 in partial accordance with Levinsen's views, according to which 

 the structure of the individual hydrothecse (especially the 

 operculum) is of principal importance. I have been reluctant to 

 discard the old classification so long as systematists give it any 

 countenance, especially in view of the fact that many of the 

 Australian species are so imperfectly known that it would be 

 impossible to classify them on Levinsen's system. Even in 

 8. elongata, our commonest species, nothing is yet known of the 

 operculum, while in various species which have been often 

 examined, competent observers like Nutting and Levinsen are 

 unable to agree as to the actual structure of this appendage. 



