CQ CTENOPHORA. 



p. 334). No Other arguments are produced for this interpretation of the axes in the paper quoted, but 

 later on further support of it is found in the shape of Heteroplaiia. (On Heteroplana, a new genus 

 of Planarians. Qu. Journ. of Micr. So. N. S. 40. 1898. p. 203). 



Concerning the remarkable Thoe paradoxa (Chun. Monograph, p. 120 — 122) I would decidedly 

 join the opinion of Clans') that "offenbar handelt es sich in jenem Ausnahmsfall lediglich um eine 

 in Folge heterochroner Entwicklung, eventuell Riickbildung und Ausfall eines Tentakels, eingetretene 

 Symmetriestorung, wie sie in ahnlichen Fallen auch an den Randfaden von Medusen nicht selten beob- 

 achtet wird. Aus derselben ergibt sich aber keineswegs die Schlussfolgerung, die der Trichterebene 

 zugehorige Achse als ungleichpolig zu betrachten und deshalb die Trichterebene im Gegensatze zur 

 Magenebene als sagittale zu orientiren und noch weniger die Berechtigung, durch diese den Leib in 

 eine rechte und linke Halfte zu zerlegen". If it were a general feature in Ctenophorans that the 

 tentacles were unequally developed, or even one of them totally reduced, the more developed tentacle 

 pointing forwards during the locomotion of the animal, being thus really a "directive" tentacle, that would 

 be a valuable argument for regarding the tentacular axis of Ctenophores as homologous with the 

 sagittal axis of bilateral animals. But the Thoe is quite an exceptional case, from which such a view, 

 contradictory to the general morpholog}' and physiology of the whole group, cannot get reasonable 

 support. — As for Heteroplana it seems to me quite unnecessary to enter on a discussion of it, the 

 more so as it is not quite easy to iinderstand Willey's exposition of its significance for the question 

 about the axial relations. It may be sufficient to state that Heteroplana is evidently only a mutilated 

 Polyclad (Comp. p. 35). 



Willey's other argument, that the paired structures of Ctenoplana are arranged about the 

 tentacular axis, I cannot regard as very forcible either — though the same arrangement is also found 

 in Tjalfiella. It seems hard to understand, how the organs could be otherwise arranged, given the 

 elongation of the animal in the tentacular axis and the shortening of the sagittal axis. If the elongation 

 had taken place in the sagittal axis, the tentacular axis being shortened, the organs would necessarily 

 be arranged about the sagittal axis. Examples of this are afforded by the Cestidse and, less markedly, the 

 Lobatae. Thus this argument for regarding the tentacular axis of Ctenophores as homologous to the 

 sagittal axis of Bilateraha seems rather valueless. That the tentacular axis is originally the longer of 

 the two horizontal axes of the Ctenophores, as appears evident from the fact that in the young of all 

 the tentaculate forms, even of the Cestidse, this axis is the longer, does not any more lead to the 

 conclusion that this axis is really the sagittal, the length of an axis being in itself of no great 

 morphological value. Especially, it is impossible to regard the greater length of the tentacular axis 

 as a proof for its being really the sagittal axi.s, when this interpretation is in contradiction with other 

 morphological facts of primary importance, as is the case here. 



Abbott (Morphology of Coeloplana, p. 62), on accepting Willey's interpretation of the axes, 



points out as a supposed further error of Lang "that he homologized the excretory tubes leading 



upwards from the infundil)ulum with the anterior branch of the gastric system of the Polyclads". 



It seems that Abbott has overlooked the embryological evidence for this homology (Comp. p. 38)=^). In 



■) C. Claus. tJber Deiopea kaloktenota Chun als Ctenophore der Adria. Arb. a. d. Zool. Inst. Wien VII. 1886. p. 12. 



2) Hatschek (Lehrbuch d. Zoologie p. 319) states that "die Entwickluugsgeschichte der Polycladen aufs klarste gegen 



die Lang'sche Auffassuiig spricht; der aquatoriale Wimperkranz der Polycladenlar\en entspricht nicht dem Korperrande, 



