CTENOPHORA. 



51 



fact, I think, this evidence rather conclusive, so it will scarcely be necessary to take up this question 

 for discussion here, the more so as Abbott does not give any reason for his assertion. On the other 

 hand, the excretory tubes afford au argument of great value in the discussion of the homologies of 

 the axes. As is well known, the infundibular canal in typical Ctenophores divides below the apical organ 

 into two branches, excretory canals, which again branch before opening outwards. (Ouly one of these 

 branches opens outwards, the other remaining a closed ampulla). The two first branches always 

 lie in the sagittal ("stomachal") plane; in Tjalfiella and Coeloplana the two, simple excretory canals 

 proceed directly from the infundibulum, the infundibular canal having been reduced on account of 

 the shortening of the longitudinal (main) axis; they lie in the sagittal plane, in conformity with what 

 obtains in other Ctenophores. That the same will prove to be the case in Ctefwplana can scarcely 

 be doubted. — It is then evident that if we can point out a homologue to the excretory vessels of 

 Ctenophores in the Polyclads, we will have a sure argument for which axis is homologous to the 

 sagittal axis of Ctenophores — and this is fortunately the case. Only one tube is developed in the 

 Polyclads, being simple as in Tjalfiella^ and having only in the early stages au indication of an outward 

 opening; of the other tube slight traces may be distinguished. Now, this excretory tube is 

 situated behind the cerebral ganglion (apical organ); in the later development it prolongs 

 forward together with the moving forwards of the brain, and develops into the tinpaired anterior 

 branch of the gastrovascular system, the position of which above the cerebral ganglion is thus 

 naturally explained (Comp. fig. 10, p. 39). This fact (which was justly emphasized by Lang) seems 

 to prove conclusively, that the longitudinal axis of the Polyclads is homologous 

 to the sagittal axis of the Ctenophores, as maintained by Lang. Therefrom naturally follows, 

 that the nuchal tentacles of the primitive Polyclads are homologous to the tentacles of Ctenophores, 

 not to the sensory tentacles, viz. polar fields of Ctenoplana, and the tentacles of Ctenophores have 

 nothing to do with the proboscis of Nemerteans, as suggested by Willey. 



The homologies of the gastrovascular system are especially pointed out by Korschelt 

 & H eider (Op. cit.) as "wenig befriedigend", and Lang (Monogr. p. 648) also acknowledges that 

 Coeloplana, in which, according to the description of Kowalevsky, the mouth opens directly into the 

 infundibulum, without an ectodermal pharynx, "entfernt sich in dieseni Punkte ebenso sehr von den 

 Polycladen wie von den Ctenophoren". The researches of Abbott have eliminated this difficulty; 

 but the study of Tjalfiella also gives additional support for the homology of the gastrovascular system 

 of Polyclads and Ctenophores. 



A detailed homologizing of the ramifying gastrovascular canals of Polyclads with those 

 of the Ctenophores cannot be carried out, at least in the present state of knowledge. In Tjalfiella there 

 is first formed a single pair of ramifying canals on each side of the sagittal axis, and from these the 

 genital canals are differentiated later on. Ctenoplana is not sufficiently known in this respect. In 

 Coeloplana the arrangement of the main gastrovascular canals appears to differ very considerably 

 from that of Tjalfiella, especially in the presence of two main canals in the sagittal plane. These 



sondern umgiirtet den Korper der Quere nach und das Cerebralganglion entsteht am vorderen Korperpol". I fail to see the 

 discrepancy between tlie developmental facts and the homologies maintained by Lang. That the cerebral ganglion develops 

 at the anterior end of the embryo, is in no way contraditory to the fact that it, together with the whole anterior end, wanders 

 forward in the course of development. 



7* 



