CTENOPHORA. 



is no reason to enter here on a discussion of the affinities of the Ctenophores to the Coelenterates 

 and their possible derivation from either the Hydromedusse (especially the Cladonemidse), the Narco- 

 medusae or the Actiniae, nor to discuss the curious view held by K. Cam. Schneider 1 ) that the Cteno- 

 phores are related to the Porifera, founded mainly on the fact that both possess "ein echtes Mesoderm, 

 das in engster genetischer Beziehung zum Ektoderm steht" (Op. cit. p. 3ay). It is the theory of the 

 affinities between Ctenophores and Polyclads, which concerns us here, on account of the light which 

 Tjalfiella throws on Ctenoplana and Coeloplana, those two forms which have played so important 

 a role in the discussions of this question. It will be necessary to give a short account of this theory 

 and its history. 



The first to express the view of a nearer relation between Ctenophores and Plauariaus was 

 Selenka, who in his "Zoologische Studien. II. Zur Entwicklungsgeschichte der Seeplanarien" (i88i) 2 ) 

 points out several important parallels in the embryological development of the two groups; he con- 

 cludes therefrom (p. 31) "dass die marinen Polycladen oder iiberhaupt die Turbellarien aus ctenophoren- 

 ahnlichen Wesen hervorgegangen seien, indem letztere aus der schwimmenden in die kriechende 

 Bewegung ubergingen". He suggests that possibly among the Rhabdocoela transitional forms may be 

 found; whether Coeloplana (which had then quite recently been described by Kowalevsky) is such 

 a transitional form "lasst sich vorlaufig nicht beurtheilen". 



Soon after the same theory of the close affinity between Ctenophores and Polyclads was set 

 forth by A. Lang, independently of Selenka, in his paper "Der Bau von Gunda segmentata und die 

 Verwandtschaft der Plathelminthen >mit Coelenteraten und Hirudineen"3), the conclusion being "mit 

 einem Worte, dass s*ie (the Polyclads) kriechende Ctenophoren sind" (p. 215). - - In his great Monograph 

 "Die Polycladen (Seeplanarien) des Golfes von Neapel und der angrenzenden Meeresabschnitte" 

 (p. 645 666)4) Lang again discusses this theory in a very detailed manner, altering his views from 

 the first paper only in some minor points. An elaborate comparison of the morphology and embryology 

 of the two groups is given as support of the theory, which appears directly confirmed through the 

 existence of an intermediate form like Coeloplana.- The theory is also discussed by Chun5) ; who 

 does not, however, take a definitive position towards it. 



Starting with the axial relations of the body of Polyclads and Ctenophores, Lang points out 

 that the main axis, which is vertical in Ctenophores, has been bent in Polyclads 6 ). In the embryos 

 of Polyclads it is vertical as in Ctenophores, the cerebral ganglion (apical organ) lying vertically over 

 the mouth; behind the ganglion is a vertical branch from the gastrovascular system representing an 

 excretory vessel. In the course of development the ganglion moves forward, entraining the excre- 

 tory vessel, which develops into the anterior branch of the gastrovascular system. Thus the position 

 of the brain below this branch is naturally explained. The important fact that in young stages there 

 is over the excretory vessel "eine Liicke im Ektoderm, durch welche das daruuter liegende Entoderm 



') K. Camillo Schneider: Histologische Mitteilungen. I. Die Urgenitalzellen der Ctenophoren. Z. wiss. Zool. 76. 1904. 



2 ) See also his note "Zur Entwicklungsgeschichte der Seeplanarien" in Biol. Centralblatt. I. 1881. p. 229239. 



3) Mitth. a. d. Zool. Stat. Neapel. III. 1882. p. 187250. Taf. XII- XIV. 



4) Fauna u. Flora d. Golfes v. Neapel. Monographic XI. 1884. 



5) C. Chun. DieVerwandtschaftsbeziehungen zwischenWiinnern und Coelenteraten. Biol. Centralblatt. II. 1882. p. 5 16. 



6) In the treatise on Giitula segmentata Lang homologized the longitudinal axis of the Polyclad body with the main 

 axis of Ctenophores, the original position of the mouth in Polyclads thus being taken to be at the posterior end of the body, 

 as is nearly the case in Cestoplana. This view is corrected in the Monograph of the Polyclads (p. 646). 



