4 8 



CTENOPHORA. 



typical Cydippid, the more so as the costae of the grown Ctenoplana are quite similar to those of 

 the young Tjalfiella. Finally, it should be emphasized that, even if it is the natural order of events 

 that the pelagic fauna has originated from littoral organisms, it is quite possible that some pelagic 

 forms may again have adopted a littoral, non-pelagic habit 



All evidence is thus against the view of Willey that Ctenoplana and Coeloflana are primi- 

 tive forms. They, together with Tjalfiella, must be regarded as highly specialized forms, derived from 

 typical, free-swimming forms, of the Mertensiid type. Willey's eloquent apology for Ctenoplana not 

 being a degenerate form cannot alter this conclusion. It may perhaps be a consolation that the three 

 forms are not maintained as "degenerate", but, on the contrary, as highly specialized forms. 



With this it seems definitely settled that the Platyctenida ("Archiplanoidea") are not the an- 

 cestors of the pelagic Ctenophores. We may then return to the original theory: the Turbellarians being 

 derived from the Ctenophores. But here another essential question must be taken into consideration, 

 viz. which group represents the most primitive of the Turbellarians, the Polyclads as maintained 

 by Lang, or the Acoela, as maintained by v. Graff. 



In his work on the Turbellaria in "Bronn. Klassen u. Ordnungen d. Thier-Reichs" v. Graff 

 expresses the view that the nervous system of the Acoela (36 pairs of equally developed longitudinal 

 stems, arranged radially round the main axis) represents a primitive condition, which disappears more 

 or less completely in the coelate Turbellarians with the adaptation to the creeping habit. "Diese Con- 

 figuration des Nervensystems weist auf die Abstammung von radiaren Formen hin und festigt die 

 Anschauung, dass die Acoleu die den Turbellarienahnen nachststehende Gruppe reprasentiren. Sie 

 macht die Annahme einer Knickiing der Hauptaxe im Sinne Lang's, sowie einer secundaren Ver- 

 schiebung des Gehirns an das heutige Vorderende der Turbellarien uberfliissig und lasst als form- 

 bildende Factoren, welche die Herausbildung der streng bilateralen colaten Turbellarien aus radiaren 

 Ahnen erklaren sollen, nur eine Verschiebung des Muudes nach der Bauchseite bei der Anpassung an 

 die kriechende Lebensweise nothwendig erscheinen" (p. 1974). "Die heutigen Acolen leite ich (v. Graff) 

 von drehrunden, langgestreckten Formen ab, mit einer nahe dem Vorderende senkrecht zur Hauptaxe 

 stehenden Gehirnplatte, deren Mitte durch die ihr anliegende Statocyste bezeichnet war . . ." (p. 1975). 

 Trichoplax is thus dropped as an ancestor of the Turbellaria, - - very appropriately, it being shortly 

 afterwards shown by K rum bach 1 ) to be only the planula of Eleutheria, probably abnormally altered 

 through aquarium conditions. 



But von Graff does not indicate from which kind of Coelenterate he thinks the Acoela 

 have been derived. The ancestor he depicts does not correspond to any type of radiate animals 

 known. It seems evident that he is at a loss to point out any radiate type from which the Acoela 

 could with any probability be derived. Further, when he sees an advantage in his theory in the 

 fact, that it makes the bending of the main axis superfluous, which according to Lang's theory must 

 have taken place, he appears to have overlooked that this bending has been shown by Lang to take 

 place ontogenetically in the development of the Polyclads; in the same way he seems to have over- 

 looked that the original position of the mouth in the Polyclads is in the middle of the underside, not 

 at the posterior end of the body, no wandering of the mouth being necessary to produce the bilateral 



') Th. Krumbach. Trichoplax, die umgewandelte Planula eiiier Meduse. Zool. Anz. 31. 1907. p. 450 454. 



