CTENOPHORA. ,7 



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

 suggest, that Tjaljiella and its nearest relations Ctenoplana and Coeloplaria, come from the Cydippids through 

 forms like Bathyctena (Mcrtcnsia) Chuni. And probabh- the Lobatse are also derived from such forms. 



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

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

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

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

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

 adopting the two families: Cfenoplaiiido' and Coeloplaiiidcr established by Willey (On Ctenoplana; 

 ]D. 341), Tjaljiella then evidently belonging to the former family. But so long as our knowledge of the 

 anatomy of Cteuoplaiia and Coeloplana is so insufficient, and their development even quite unknown, 

 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 much 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 3). I may refer to the remarks of Chun (Monograph, p. 245 — 

 256). So far as I know, this theory 4) has not been adopted since then by anybody. Likewise there 



") L. Agassiz. Contributions to the Natural History of the Acalephae of North America. Part II. On the Beroid 

 Medusse of the shores of Massachusetts, in their perfect state of development. 1849. (p. 366). 



2) A. Agassiz. North American Acalephje. (111. Cat. Mus. Comp. Zool. II. 1865. p. 11 — 12); Embrj'ologj- of the Cteno- 

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



3) E. Metschnikof f. Studien iiber die Eutwickluug der Siphonophoren und Meduseu. (Zeitchr. f. wiss. Zool. XXIV. 

 1874. p. 70—77)- 



4) The main point of this theory is the homologiziug of the gastrovascular canals of Ctenophores with the ambula- 

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

 vesicles in tlie young Echinoderni larva is found to resemble that of the entoderm -~ the (ectodermal) pharj'nx in the young 

 Ctenophore, this resemblance forming the main proof of the theorj-. The Ctenophores are regarded as "prophetic animals" 

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

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

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

 system, yet in the joung larvas we can see that this connection exists, the water system being formed by diverticula from the 

 digestive cavity". .A. Agassiz: North American .\caleph8e. p. 11 — 12). 



