CTENOPHORA. 



45 



Ctenoplana — is in good accordance herewith ; such a membrane is not developed in t)-pical Ctenophores. 

 Also the nervous system appears to be specially developed, as expected by Lang; indeed Abbott 

 has found four distinct ganglia developed round the otolith (Op. cit. p. 6i. Fig. B); also in Ctenoplana 

 similar ganglia are described by Korotneff. 



While Abbott is thus opposed to the view of Willey that Coeloplana and Ctenoplana are 

 primitive forms, he does not, however, accept Lang's theory either. Besides the erroneous homo- 

 logizing of the axes pointed out by Willey, he finds another important error in Lang's theory, viz. 

 the homologizing of the excretory tubes of Coeloplana with the anterior branch of the gastrovascular 

 system in Polyclads (Op. cit. p. 62). Otherwise he does not express any definite opinion regarding the 

 phylogenetic questions, stating only that until the development of Coeloplana has been worked out 

 "the true position of Coeloplana and its relationship with other groups cannot be certainly decided". 



(p. 66). 



A series of papers discussing the systematic position of the Cteuophora, their relation to the 

 Polyclads a. o.. by Kemna, Lame ere and Schouteden') may briefly be mentioned here. They 

 are mainly speculative, adducing no new facts. Kemna is inclined to adopt Lang's theory, though 

 laying stress on the Polyclad-larvae, the processes of the "Miiller's larva" being compared with the 

 lobes and auricules of the Lobata. — Lameere in his first paper maintains the opinion, also held 

 by V. Beneden, that the Ctenophores are Polyclads which have adopted a pelagic mode of life; in the 

 second paper he is of opinion that the discovery of Hydroctcna has fixed the position of Cteno- 

 phores as "Hydrozoaires", not as pelagic Turbella- 

 Hydroctena rians; they are "le dernier terme de revolution des 



Narcomeduses", and the Polyclads have nothing 

 to do with them. — Schouteden, who is equally 

 convinced of Hydroctena representing the ancestral 

 form of Ctenophores, gives the present diagram 

 of the relations of the Ctenophores (- that 

 Pseudocoeloplana rests on a misunderstanding has 

 been pointed out above, p. 31 — ). 

 Finally, in 1911 B. Hatschek in his pamphlet "Das neue zoologische System" (p. 8— 9) gives 

 the important statement that Metschnikof f's description of the formation of the mesoderm in 

 Ctenophores is "durchaus irrig". The whole mesoderm "entsteht nur von der Umgebung des Mundes 

 durch Absonderung einzelner Ectodermzellgruppen", in accordance with the observations of Ko wa- 

 le vsky^). He does not enter specially on the theory of the derivation of the Polyclads from the 

 Ctenophora; judging, however, from the genealogical tree which he gives (p. 18), he evidently accepts 

 the theory, (as he did previously in his "Lehrbuch d. Zoologie"), the "EcterocoeHa" (among which the 

 Proscolecida, including the Platodes, rank as the lowest group) being derived from the "Ctenozoa". 

 ■) Ad. Kemna. Sur les rapports entre Ctenaires et Polyclades. Ann. Soc. R. Zoolog. et Malacol. de Belgique. 38 1903. 

 p. LXXIX— LXXXVII. 



Aug. Lameere. L'Origine des Ctenophores. Ibid. p. LXXXVII— XCVI. - Ctenophores et Polyclades. Ibid. 40. 1905. 

 p. CXXVIII-CXXX. 



H. Schouteden. Les affinites des Ctenophores et Polyclades. Ibid. 40. 1905. p. CXVIII— CXXVII.^ 



2) A. Kowalevsky. Entwickelungsgeschichte der RippenquaUen. Mem. Acad. St Petersbourg. 7. Ser. X. 1866. 



Ctenophores Ctenoplana 



-Pseudocoeloplana 



Coeloplana 



I . • 



Polvclades 



