CTBNOPHORA. 



37 



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

 suggest, that TjalficllaanA. its nearest relations CfenoplanaanA Coeloplana, come from the Cydippids through 

 forms like Bathyctena (Mertensia) Chum. And probably 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 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: Ctenoplanidce and Coeloplantdce established by Willey (On Ctenoplana; 

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

 anatomy of Ctenoplana 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 

 Lobatie 



Pleurobrachiidse 



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 1 ) and later on carried out in more detail 

 by A. Agazziz 2 ) 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 

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



2 ) A. Agassiz. North American Acalephae. (111. Cat. Mus. Comp. Zool. II. 1865. p. 11-12); Embryology of the Cteno- 

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



3) E. Metschnikoff. Studien fiber die Entwicklung der Siphonophoren und Medusen. (Zeitchr. f. wiss. Zool. XXIV. 

 1874. p. 70 77). 



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

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

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

 Ctenophore, this resemblance forming the main proof of the theory. 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 alimentary canal, 

 in Ctenophone, 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 ambulacra] and the digestive 

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

 digestive cavity". (A. Agassiz: North American Acalephse. p. n 12). 



