CXENOPHORA. 41 



aus Stiahlthieren — ein Postulat der neueren Morphologic — in der am meisten befriedigenden Weise 

 erklart, indem sie sich ebenso sehr aiif den Thatsachen der vergleichenden Anatomic und Ontogenic, 

 als auf biologische und physiologische Erwagungen stiitzt, und nicht eine unabsehbare Reihe unbe- 

 kannter hypotetischer Zwischenformen . . . erfordert". On the other hand he does not conceal the fact 

 that there are as yet considerable difficulties to the theory. These are mainly the excretory system and 

 the development of the mesoderm ; also the homology of the nervous system is somewhat less satisfactory. 

 — Regarding the excretory system Chun has suggested that the rosettes of Ctenophores may possibly 

 be the homologues of the excretory cells of Polyclads; Lang, however, is not very inclined to think this 

 correct, as he has never found the flame-cells lying in the epithelium of the gastrovascular system in 

 Polyclads (— though this is found in Gunda segmentata — ), and the rosettes are not connected with 

 ducts opening to the exterior. For the mesoderm Lang gives some theoretical considerations 

 (Monogr. p. 660) which might seem to reconcile the discrepancies on this point, but he does not him- 

 self seem very satisfied therewith. The solution of these questions must be left for future researches. 



Not less important would be the close study of Coeloplana, known at that time only from 

 Kowalevsky's short notice, which gives no information about its nervous system, musculature, 

 genital or excretory organs. Its branching and anastomosing gastrovascular canals correspond with 

 those of the Polyclads, but there is no mention of a pharynx; if this is really lacking, "so entfernt 

 sich Coeloplana in diesem Punkte ebenso sehr von den Polycladen wie von den Ctenophoren" (p. 648). 

 "Ein unabweisbares physiologisches Postulat ist ferner das, dass bei Coeloplana die Anpassung an die 

 kriechende Lebensweise eine grosse Veranderung in der Anordnung der Muskulatur und damit des 

 motorischen Nervensystems nach sich gezogen habe. Da die Rippengefasse fehlen, miissen ferner auch 

 die Geschlechtsorgane anders angeordnet sein als bei den Ctenophoren" (p. 650). It would be equally 

 interesting to learn, whether perhaps copulatory organs have been formed. 



It did not last long before this Selenka-Lang's theory, which really looks so very attractive, 

 got important support from other sides. In 1885 Metschnikoff published his researches "Uber die 

 Gastrulation und Mesodermbildung der Ctenophoren" i), in which he shows that the mesoderm is not 

 formed by cells wandering in from the ectoderm, as was previously supposed, but as a separate for- 

 mation, being divided off from the oral pole of the (entodermal) macromeres and transferred during 

 the- further development to the aboral pole, where it develops into four radiating thickenings. Thus 

 the formation of the mesoderm is found to be essentially in accordance in the two groups. 



Already the next year another splendid confirmation of the theory appeared in the discovery of 

 Ctenoplana, through Korotneff. The phylogenetic importance of this remarkable new form is 

 fully appreciated by Korotneff, who thus summarizes his view of Cte?ioplana and Coeloplana: they 

 are "zwei Ubergangsformen, welche an verschiedenen Seiten der Scheidelinie zwischen den Cteno- 

 phoren und Planarien stehen: die Ctenoplana neigt sich mehr den Ctenophoren, die Coeloplana den 

 Planarien, obschon die beiden Formen zu derselben Zeit sehr nahe verwandt sind"^). The relations of 

 Ctenoplana to the Ctenophores on the one side, and to the Polyclads on the other, are in the main 

 these: The flattened body in which a dorsal and a ventral side are differentiated (as in Coeloplana), 



I) E. Metschnikoff. Vergleicheiid embryologische Studien. Zeitschr. f. wiss. Zool. Bd. 42. p. 648— 656. 

 -) Zeitschr. f. wiss. Zool. Bd. 43. p. 249—250. 



The Ingolf Expedition. V. 2. 



