30 



CTENOPHORA. 



lining of large cells recalls the prominent part of the ovary in Tjalftella, shown e. g. in PI. IV, Fig. 3, 

 the left genital organ on the npper side of the figure. I do not venture, of course, to maintain that 

 this "genital coecum" really represents the ovary, but I must maintain that it cannot be considered 

 as definitely proved that the genital organs of Ctenoplana are unisexual, and for my own part I think 

 it very probable that they will prove to agree completely with those of Tjalficlla in regard to the 

 presence of both testis and ovary in the same organ. 



Another point in the structure of the genital organs of Ctenoplana is that they are surrounded 

 by a tunica propria. In Tjalficlla I have been unable to find anything which might be termed thus, 

 and it is also totally unknown in other Ctenophores. As Wil ley's figures are, upon the whole, not 

 very elaborate, I confess that I cannot feel quite convinced of the existence of this tunica propria; in 

 any case, it seems to me very desirable to have this point made the object of further researches. 



The most remarkable statement about the genital organs in Ctenoplana is, however, that they 

 open to the exterior through one or more ducts each. Though the figures 7 — 9 of Willey's paper 

 seem to show this to be really the case, I would recall the well known ectodermal invagina- 

 tions over the genital organs in Calliaiiira, and especially those of Tjalfiella. In these two cases 

 it is beyond doubt that they have nothing especially to do with the genital organs. The fact that 

 Ctenoplana is the only Ctenophore known which appears to have such genital ducts, which are thus a 

 quite unparalleled feature among Ctenophores, makes it, at least, very desirable to have the matter 

 reexamined. — If they are really genital ducts, it ma)' perhaps be explained as a special adaptation 

 of a structure homologous to the invagination over the genital organs existing in Tjalficlla. 



Having thus shown that the apparent great differences in the gastrovascular system and 

 muscular system between Ctenoplana and Tjalfiella are due to misconceptions in the description of 

 Ctenoplana, and that there is likewise very probably fair accordance between their genital organs, 

 at least in the main points, I think it evident that these two forms are indeed nearly related, 

 the differences being mainly of adaptive nature, due to the different habits of the animals. The dis- 

 appearance of the costse, the rudimentary condition of the apical organ and the development of the 

 peculiar chimneys in Tjalfiella are quite evidently adaptations to its sessile habit. Finalh', the lacking 

 of the sagittal lobes of the gastrovascular system in Tjalfiella is in accordance with the compressed 

 shape of its body. Differences not to be thus explained are: the structure of the tentacles (unbranched 

 in Tjalfiella, branched in Ctcnoplana\ the character of the peripheral canals of the gastrovascular 

 system (anastomosing in Ctenoplana, simply branched in Tjalfiella), the existence of only four genital 

 organs in Ctenoplana, while Tjalfiella has eight. Also the presence of genital ducts in Ctenoplana, 

 if it proves to be a fact, is a feature not to be explained as an adaptation to the habit of animal. These 

 differences are, however, not of primary importance. That Ctenoplana and Tjalfiella form two very 

 well distinguished genera nobody, I think, will deny; but I think it equally evident that these two 

 genera, in spite of their very different appearance, are nearly related and belong to 

 the same family. 



The anatomy of Coeloplana is not sufficiently known for a detailed comparison with Tjalfiella, 

 but through Abbott's very important paper "The morphology of Coeloplana" (Zool. Jahrb. Abt. f. Anat. 



