^8 CTENOPHORA. 



as to the entodermic origin of the genital cells, but m the Monograph on the Polyclads he has some 

 doubt about this. Since, however, in the Coelenterates the genital cells may arise either from the 

 ectoderm or the entoderm in nearly related forms, Lang is of opinion that such a discrepancy woiild 

 not imply any considerable difficulty to the theor}-. This may perhaps be true; but it would certainly 

 be more satisfactory to the theory, if it could be proved that the origin is the same in both groups — 

 and the new facts acquired are decidedly in favour thereof. As has been shown above, all evidence 

 is decidedly for the entodermal origin of the genital cells in Ctenophores ( — and Tjalfiella is not the 

 least important support for this — ); regarding the Polyclads the observations of Lang are equally 

 in favour of the entodermic origin. So long then as there is no proof of the contrary, it may be held 

 that there is accordance in this point between the two groups. — A difference not to be removed, at 

 present at least, is this, that while in the Ctenophores the genital products are ejected through the 

 gastrovascular system, no genital ducts existing, in the Polyclads there are special genital ducts, the 

 genital organs being lined with a tunica propria which continues directly into the genital ducts. 

 According to the statements of Willey Ctenoplana would afford a very important transitional stage 

 in this regard, the genital organs being lined with a tunica propria continuing into genital ducts 

 opening outwards, on the dorsal side. But, as pointed out above (p. 29 — 30), these statements very much 

 need to be verified. On the other hand it is by no means improbable that Coeloplana, the most spe- 

 cialized of the creeping Ctenophores, will prove to be intermediate between Ctenophores and Polyclads 

 in regard to the genital organs; it would even not seem very surprising if copulatory organs were 

 developed in Coeloplana, as is suggested by Lang. In any case it will be of the greatest interest to 

 have the genital organs (and development) of Coeloplana examined. 



The greatest difficulty to the theory lies in the excretory organs, as is duly pointed out by 

 Lang. It has been suggested by Chun that po.ssibly the rosettes of the Ctenophores are the homo- 

 logues of the excretory organs; but there is, in any case, no proof of this. But even if nothing can 

 be found in Ctenophores corresponding to the excretoi)' organs, it may be said of these as of the 

 copulatory organs, that they must have originated once, and then it is only what should be expected, 

 that they have originated in the most primitive of the Bilateralia. The suggestion that Coeloplana 

 might possess an excretory system (Lang, p. 655) seems disproved by Abbott's discovery that it 

 has typical ctenophoran rosettes. 



Finally the question of the mesoderm must be mentioned. Through the researches of 

 Metschnikoff a complete accordance appeared to be established between Ctenophores and Poly- 

 clads in this respect, and, as Samassa states (Op. cit. p. 235) "dieselben waren sogar allein im Stande 

 die von den iibrigen Bilateralthieren so sehr abweichenden Mesodermverhaltnisse der Polycladen auf- 

 zuklaren". The very definite assertion of Hatschek ("Das neue zoologische Sj'stem", p. 8) "dass die 

 Angaben Metschnikoff's iiber die Bildung des Mesoderms dieser Tiere durchaus irrig sind" would 

 seem to raise new difficulties here. Hatschek's researches being as yet unpublished, it is impossible 

 to enter on a discussion of this question. But the fact pointed to above (p. 45) that Hatschek 

 also derives the Turbellarians from a point on the stem of the "Ctenozoa", seems to show that the 

 development of the mesoderm in the Ctenophores is not opposed to the theory of Lang. 



Having thus discussed the different points of the Selenka -Lang's theory to which objections 



