42 CTENOPHORA. 



is a Planarian character, as is also the general ciliation (— the existence of ciliation on the dorsal 

 side being, however, denied later by Willey — ); the presence of costse, on the other hand, is decid- 

 edly a Ctenophoran character. The gastrovascnlar system is in accordance with that of the Polyclads 

 (and Coeloplana) throngh the branching and anastomosing of the peripheral canals; on the other 

 hand, it differs from both Ctenophores and Polyclads in lacking a pharynx — Korotneff having 

 qnite misunderstood the structure of the pharynx, (comp. p. 25). Also in the musculature he 

 finds a great difference from both Ctenophores and Planarians — "aber es wird kanm bestreitbar 

 sein, dass die Muskulatur am meisten den ausseren Einfliissen widerstehen und deswegen schreibe ich 

 (Korotneff) den Eigenthumlichkeiten dieses Systems bei der Ctenoplana keine besondere Wichtigkeit 

 (genetisch) zu". — It is curious to see, how Korotneff tries to remove the apparent great difficulty 

 arising from the remarkable muscular system, which he has described; the difficulty is certainly better 

 removed by the demonstration that his "muscular system" rests on a complete misconception, (comp. 

 p. 28—29). 



In .spite of these misconceptions and though no information is g^ven of so important a struc- 

 ture as the genital organs the view of Korotneff that Ctenoplana represents, as well as Coeloplana, 

 a transitional form between Ctenophores and Polyclads appears well enough founded. Further support 

 for Lang's theory is given by Samassa in his paper "Zur Histologie der Ctenophoren" p. 235 — 238. 

 He there points out that the histology of Ctenophores and Polyclads is even more in accordance than 

 supposed b)' Lang; also the homologizing of the nervous system in the two groups he finds fully 

 justified. He further maintains that the Polyclads are decidedly the most primitive of the Turbellarians 

 against L. v. Graff, who in his work "Die Organisation der Turbellaria Acoela" 1891 (p. 49-52) 

 expresses the opinion, against "die unbewiesene Ansicht dass die Turbellarien (mit den Polycladen als 

 Wurzel) von den Ctenophoren abstammen", that the Acoela are the most primitive of Turbellarians, 

 the Acoela being derived from such undifferentiated forms as Trichoplax. — Also later on (in Broun. 

 Klassen u. Ordn. d. Tierreichs. Bd. IV. Abt. I. C. Turbellarien. 1904— 1908) v. Graff decidedly main- 

 tains the same opinion, with the exception that Trichoplax is not here regarded as an ancestral form 

 of the Turbellarians. 



The theory of the derivation of Polyclads from Ctenophores thus apparently rested on a much 

 firmer basis, than when it was worked out by Lang; for a rather long time it remained nearly") un- 

 disputed, until in 1896 Willey turned the whole theory upside down 2). 



In discussing the theory Willey first points out that the axial relations between Ctenophores 



and Polyclads, as represented by Lang, rest on a misconception. "Ctenoplana unequivocally proves, 



as I (Willey) think, that the tentacle plane or funnel-plane of it and the Ctenophores corresponds to 



the sagittal plane of bilateral animals, and not to the transverse plane" (p. 332). Chun, who was at 



first in doubt as to the criterion by which to homologize the planes of Ctenophores with those of the 



Bilateria, came through the discovery of the peculiar Thoe paradoxa. in which at first onh- one 



("directive") tentacle is developed, to the conviction that the tentacle axis is homologous to the long 



axis of Bilateria. Through regarding Ctenoplana Willey comes to the same conviction. "In Cteno- 



') Comp. p. 46, the remarks on Korschelt & Heider's (Lehrbuch d. vergleicheiideii Entwicklungsgesch. d. wir- 

 bellosen Thiere) and Hatschek's (Lehrbuch d. Zoologie) position towards the theory. 

 2) A. 'Willey. On Ctenoplana. (Quart. Jouni. Micr. Sc. N. S. 39>. 



