CTENOPHORA. 



37 



known as yet is the one showing most resemblance to Tjalfiella ; it would then not seem unreasonable to 

 suggest, that Tjalfiella and its nearest relations Cfenoplafm and Cocloplana, come from the C3'dippids through 

 forms like Batliyctena (Mcrtcnsia) CJiuui. And probably the Lobatte are also derived from such forms. 



While the three aberrant Ctenopliores thus seem to come from the Cydippids, they are, of course 

 so much specialized that it is impossible to unite them with this order. Thev evidently form an 

 order for themselves, the Platyctenida. Whether the\- should also be united into one family, cannot be 

 decided at present. It seems evident that Ctenoplana and Tjalfiella are the most nearly related of 

 the three, while Coeloplana would seem to stand more apart; thus far there would be no difficulty in 

 adopting the two families: Ctcnoplaiiida and Coeloplaiiidtr established b\- Willey (On Ctenoplana; 

 p. 341), Tjalfiella then evidently belonging to the former family. But so long as om- knowledge of the 

 anatomy of Cfeiioplana and Coeloplana is so insufficient, and their development even quite iniknown, 

 the question of the families must be left undecided. 



The probable interrelations of the tentaculate Ctenophores may be graphically expressed as follows: 



Platyctenida 

 Lobatse 



Pleurobrachiida; 



Cestidse 



Bathyctena 



Cydippida 



E. Phylogeny. 



The demonstration that the Platyctenida are the most specialized of all Ctenophores, instead 

 of the most primitive, has a very important bearing on the miich discussed question about the rela- 

 tion between Ctenophores and other groups of animals, especially the Planarians. 



It seems unnecessary to enter on a discussion of the theory of the affinities between Cteno- 

 phores and Echinoderms, as first expressed by L. Agassiz') and later on carried out in more detail 

 by A. Agazziz-) and Metschnikoff 5). I may refer to the remarks of Chun (Monograph, p- 245 — 

 256). So far as I know, this theory*) has not been adopted since then by anybody. Likewise there 



■) L. Agassiz. CoiitribiUions to the Natural Histon- of the Acalephre of North America. Part II. On the Beroid 

 MediiBie of the shores of Massachusetts, iu their perfect state of development. 1S49. (p. 366). 



-) A. Agassiz. North .\merican Acalephte. (111. Cat. Mus. Conip. Zool. II. 1S65. p. 11— 12); Embr3-olog>' of the Cteno- 

 phora- (Mem. Amer. Acad. X. 1874, p. 384 — 387 ; Embryology of the Starfish (Mem. Mus. Coinp. Zool. V. 1877. p. 83). 



J) E. Metschni kof f. Studien fiber die Eutwickluug der Siphoiiophoreu uud Medusen. (Zeitchr. f. wiss. Zool. XXIV. 

 1874. p. 70-77)- 



4) The main poiut of this theory is the homologizing of the gastrovascular canals of Ctenophores with tlie ambula- 

 cral vessels of Echiuodenns. The configuration of the entoderm with the protruding, but not yet separated off, enterocoel 

 vesicles in the young Echinoderm larva is found to resemble that of the entoderm -- the (ectodermal) pharynx in the young 

 Ctenophore, this resemblance forming the main proof of the theory. The Ctenophores ai-e regarded as ''prophetic animals" 

 which explain "the separation of the digestive cavitv into two distinct parts". "The separation of a sort of alimentary canal, 

 in Cteuophone, from the rest of the digestive apparatus, exactly corresponding to what exists in Echinoderm larvae; .. . although 

 in the adult starfish, or Sea-urchin, or Ophiuran, there is no apparent connection between the ambulacral and the digestive 

 s\-stein, yet in the young larvae we can see that this connection exists, the water system lieing formed by diverticula from the 

 digestive cavity". i,A. Agassiz: North American Acalephse. p. 11 — 12). 



