46 



CTENOPHORA. 



In concluding this historical review of the theory the position adopted towards it in the greater 

 hand-books may still be mentioned. Korschelt & Heider (Lehrbuch d. vergl. Entwickhmgsgesch. 

 d. wirbellossen Thiere. I. 1890) regard the Ctenophores as representing "einen nach einseitiger Richtung 

 selbstandig ausgebildeten Seitenast des Stammbaumes, der wohl kaum zu einen directen Weiterbildung 

 hoherer Thierformen die Grundlage abgab" (p. 101). Ctenoplana and Coeloplana are not regarded as 

 transitional forms between Ctenophores and Polyclads, their accordances with the latter resting "auf 



/ 



blosser Analogic"; they may show "wie sich der Uebergang freischwimmender Radiarthiere in krie- 

 chende bilaterale Formen vollzogen haben konnte" (p. 115). It is conceded that the development of 

 Ctenophores and Polyclads agrees in several important points; also the peculiar movements of the 

 cilia on the processes of Polyclad-larvse is thought of importance. But "die vielfach vorgenommene 

 Vergl eichung der Organsysteme von Ctenophoren und Turbellarien, zumal diejenige des Gastrovas- 

 cularapparats . . . ist wenig befriedigend". . . . "Selbst wenn sie aus einer einheitlichen Wurzel hervorgin- 

 gen, haben (sie) sich so stark verandert, dass die Vergleiche nur allgemeinerer Natur sein konnen (p. 115). 



Hatschek (Lehrbuch d. Zoologie. Lief. III. 1891. p. 319 332), while acknowledging "den Grund- 

 gedanken dieser Hypothese in seiner grossen Tragweite", objects to several of the homologies main- 

 tained by Lang, and specially to the derivation of the dorsoventral axis from the primary axis of 

 Ctenophores. Coeloplana and Ctenoplana are regarded as aberrant Ctenophores, not as transitional 

 forms to the Polyclads. 



G. C. Bourne (in Ray Lankester's "Treatise on Zoology"; II. 1900; Ctenophora) maintains 

 that "in point of fact we have no evidence as to whether Ctenoplana or Coeloplana are primitive or 

 derived forms; such evidence can only be furnished by their development and larval history". "In 

 the present state of our knowledge it cannot be said that the existence of Ctenoplana and Coeloplana 

 gives any satisfactory evidence of the relationship of Platyhelminthes to Ctenophora, still less of the 

 descent of the former group from the latter. The most that can be said is that Ctenoplana and 

 Coeloplana afford an interesting suggestion as to how the Polyclada might conceivably have been 

 derived from a Ctenophore-like ancestor". More weight is ascribed to the embryological points of 

 resemblances between the two groups. "The conclusion is that the Turbellaria, the Nemertines, and 

 the Ctenophora are descended from a common ancestor which is most nearly represented by the larva 

 of Stylochus". (p. 19). W. B. Benham, who has treated the Platyhelmia in Ray Lankester's 

 "Treatise on Zoology" IV. 1901, does not enter on a discussion of the relations between Polyclads and 

 Ctenophores, referring to Bourne in this connection. He only states (p. 3) that "no doubt the ancestral 

 form was more or less closely connected with the Coelentera by means of animals of which we know 

 nothing". It thus appears evident that he is not in favour of Lang's theory. It is worth mentioning 

 in this connection that the Rhabdocoela are regarded as the most primitive of Turbellaria, though 

 the nervous system is "much more highly differentiated than in the Polycladida" (p. 12); the Acoela, 

 regarded by v. Graff as the most primitive, are stated to "present every evidence, anatomically as well 

 as embryologically, of degeneration". - - F. W. Gamble, in his record of the Flatworms and Mesozoa 

 in "The Cambridge Natural History" II. 1901, p. 28, states that "the work of the last decade has 

 neither proved nor disproved Lang's suggestion that the Ctenophores and Polyclads have been derived 

 from common ancestors". 



