HYDROIDA II 



95 



mon to Syntheciidm and Sertulariida is also the exterior ectoderm lamella, which forms an inner wall 

 covering to the hydrothecse; a parallel formation is, it would seem, found only in Aglaopheniida, 

 where it is less pronounced. From the anatomical agreement of the polyps in the two families, and 

 the fundamental differences in the structure of the polyps as compared with the remaining thecaphores, 

 I have found it most correct to take the two families together in a distinct family series Sertulariina. 



With regard to division into genera, however, I can on the whole agree with Levin sen, as 

 will be seen from the following. Nevertheless it should be noted that the generic subdivision is still 

 far from satisfactory. A considerable amount of anatomical investigation will be required before we 

 can obtain a thoroughly adequate foundation here, and the present material, unfortunately, is not suit- 

 able for such studies. 



In face of the above diagnosis, it may be objected that a genus such as Diphasia lacks the 

 blind sack, at any rate, appears to do so. This point is not without importance, and has its own parti- 

 cular interest in consideration of the genus from a phylogenetic point of view. On closer examination 

 of the group of species which Levin sen classes together in the genus Sertularclla, we find that 

 there are here, as a matter of fact, two distinct anatomical groups, which ought really to be separated 

 as independent genera. The more restricted genus Sertularclla will then comprise the species in which 

 a closing apparatus consisting of 3 or 4 flaps is found together with polyps having fully developed 

 abcauline blind sack; this class includes the great majority of species. On the other hand we have 

 a group of species with the same sort of closing apparatus, but with polyps in which the blind 

 sack is rudimentary or entirely lacking as far as can be seen from external observation. For this 

 group, then, a special genus should be established. The group in question exhibits a close affinity 

 to Thyroscyphus, which again forms a link with Campanulinida^ and from this genus we must pre- 

 sume that the entire family has its origin. A species such as Sertularclla tamarisca (Linne) belongs 

 to this group of the Sertularclla, and represents the transition stage to genera with rudimentary blind 

 sack, Diphasia and Dynamema. From the second group, again, are derived the remaining genera with 

 their marked lines of development in the closing apparatus. Taking into consideration both the organ- 

 isation of the hydranth, and the structure and development of the closing apparatus, we come to fixed 

 lines of development within the family, which are best illustrated by the following scheme: 



Odontotheca Hydrallmania Thujaria 



\ ./ 



Sertularia 



Diphasia Dyiiamciia Abictiuaria 



\ / \ 



a. Polvps with rudimentary blind sack. - Si ■'/ ' lularclla - b. Polyps with blind sack. 



Thyroscyph us 



