CTENOPHORA. 



57 



inside of the lobes is clad with a thick, ciliated epithelium (comp. the sections, PI. X); in the Polyclad- 

 larvse the processes are likewise originally clad with a uniform ciliation, which in the course of 

 development becomes speciahzed so as to form a ciliated band. (Lang, Monograph, p. 377). Hat- 

 schek (Op. cit. p. 320) thinks the "praorale Wimperkranz" of the Polyclad-larvae to have originated 

 from the costse of Ctenophores through such a form as Charistephane. "Man konnte den Wimperkranz 

 von acht einander genaherten Plattchengruppen ableiten, aber umgekehrt auch den geschlossenen 

 Wimperkranz fiir das primare halten; vielleicht ist es rich tiger beide Bildungen von einem gemein- 

 samen Grundtypus abzuleiten". Without entering on a detailed discussion of this theory I would only 

 point out, that it does not seem very appropriate to derive an evidently little differentiated structure 

 as the ciliated band of the Polyclad-larvae from such a highly differentiated structure as the combs of 

 Ctenophores, and even to adduce one of the most specialized forms, like Charistephane (with only two 

 rows of exceedingly broad combs) for the comparison. Further I would suggest that it is scarcely 

 correct to designate the ciliated band of the Polyclad-larvae as a preoral band (as is also done by 

 Lang, comp. his diagram, fig. 34. B.; Monogr. p. 404). As mentioned above, Lang was sometimes in 

 doubt whether the band of the anterior, unpaired process was really in connection with that of the 

 other processes (Monogr. p. 380). This fact is in good accordance with the explanation here set forth 

 of the origin and homology of the processes and the ciliated bands of the Polyclad-larvse, viz. that 

 the processes correspond to the transversal lobes of Tjalfiella and Ctenoplana, (and 

 accordingly also to the lobes of the Lobatse), the bands being only a specialization of the 

 general ciliation covering the lobes; accordingly it is only the band of the anterior 

 ventral, unpaired lobe, which corresponds to the preoral ciliated band of the Trocho- 

 phora, the band of the paired, posterior processes corresponding to the postoral band. 

 Thus the existence of both a preoral and a postoral ciliated band, and of a non- 

 specialized ciliated part between these bands, in the Trochophora is very naturally 

 explained. 



Regarding the musculature it has already been mentioned that in Coeloplana it has been 

 specially developed in accordance with the creeping habit of the animal. That it is also more devel- 

 oped than usual in Ctenoplana is quite probable — though the enormous muscular system ascribed to 

 it by Korotneff and partly by Willey has proved to rest on a misapprehension (comp. above, p. 28). 

 Lang points out the existence of a basal membrane in Polyclads, serving as a sort of skeletal support 

 for the muscles, as an adaptation to their creeping life habit. It is very interesting to notice that 

 also in the creeping Coeloplana and Ctenoplana a basal membrane has developed, while in the sessile 

 Tjalfiella and the pelagic Ctenophores no such membrane occurs. The musculature is thus in very 

 beautiful accordance with the theory. 



The homology of the genital organs is supported by the fact that both Ctenophores and 

 Polyclads are hermaphrodites, the genital organs being in close relation to the gastrovascular system. 

 On the other hand there would seem to be a considerable difference as to how they originate. In 

 Ctenophores they are stated to originate from the entoderm (Chun) or the ectoderm (Her twig), in 

 Polyclads they originate probably from the entoderm; but this is not beyond doubt, — it is possible 

 that they are derived from the mesoderm. In the memoir on Gunda segmentata Lang was not in doubt 



Q 



The Ingolf.Expedition. V. 2. " 



