CTENOPHORA. 6l 



Mertensia ovitiii (Fabr.) F. Romer. Die Ctenophoren. Fauna Arctica. III. 1903. p. 70, 72. 



— — Fabr. F. Moser. Die Ctenophoren der Deutschen Siidpolar-Expedition. Deutsche 



Siidpolar Exp. 1901 — 1903. Bd. XI. Zoologie Bd. III. 1909. p. 123. 

 Callianira compressa Mertens. F. Moser. Ibid. p. 138. 



Non: Mertensia ovum. H. B. Torrey. The Ctenophores of the San Diego Region. Univ. Calif. 



Publ. Zool. 2. II. 1904. PI. I. I. 



Our knowledge of this species is very incomplete, in spite of its having been mentioned 

 often enough in literature and likewise repeatedly figured. This insufficiency, which is the more 

 unfortunate as Mertensia ovum is supposed to represent one of the most primitive of all Cteno- 

 phores, is mainly due to the great difficulty of preservation, so great that Romer even states 

 that it is upon the whole impossible to preserve it. This is certainly exaggerated; since it has proved 

 to be possible to preserve satisfactorily so delicate a form as Bolina infundibulum , it will doubtless 

 also be possible to do so with M. ovum; only experiments are needed in order to find out the right 

 method to be applied. Another reason of the incomplete knowledge of this form is the fact, that it 

 Occurs only in the purely arctic waters, thus being not very accessible for study, except at the Coasts 

 of North America, where it has, however, not been made the object of study since A. Agassiz 

 (Op. cit.) published his observations on its postembryonal development. 



In some Plankton-samples from the "Ingolf I have found several specimens, which can with 

 certaintj' be referred to M. ovum. The samples having been simply preserved in alcohol, these speci- 

 mens are, of course, not in a fine state of preservation; of most of them only the main parts of the 

 gastrovascular system and the tentacle bases are preserved, but a few small specimens are nearly 

 complete, though evidently much shrunk and having lost their transparency. This material, poor 

 enough, it is true, has however enabled me to give some additional information of its anatomy and 

 to correct some errors in the previous descriptions. 



Agassiz (Op. cit. p. 26) points out as a feature characteristic of this species (and genus) "the 

 great distance at which the lateral chymiferous tubes (the pharyngeal vessels) are placed from the 

 digestive cavity, and the close connection which is shown there to exist between the tentacular appa- 

 ratus and the lateral tubes, the base of the tentacular apparatus seeming to give rise to this long, 

 slender tube, enclosing the digestive cavity in its two wide arches, when seen from the broad side" 

 — this description being also given by Moser (Op. cit. p- 124). Further the tentacular apparatus 

 "differs from that of Pleurobrachia in being limited to the abactinal part of the spherosome, and not 

 extending towards the actinostorae, as in Pleurobrachia" (p. 28). The figures which accompany 

 Agassiz' description are mere sketches, which do not give much support to the description; it is 

 true that the figures 36—37 show a quite short tentacle-basis, but these figures differ so much from 

 the fig. 29, which represents a grown specimen, that it seems not very convincing that they really 

 represent the same species — though Agassiz states that in this stage "the development of the 

 actinal part of the spherosome has become so striking, that we cannot fail to recognize in the young 

 Acaleph a Mertensia". In reality no arguments are given for the referring of the young stages 

 figured to M. ovum. It is, in any case, very unfortunate that the size of the specimens figured is not 



