STRUCTURE AND DEVEU^TMENT Ol'' COELOPLANA. 35 



Moreover, both the duct and the covering membrane are formed by 

 cells showing no definite feature to distinguish them from the spermato- 

 gonia. This fact apparently shows that both the duct and the membrane 

 are derived from the spermatogonia! cells. 



In the testes that contain some spermatozoa, especially in those in 

 which the greater part of the sexual cells have been transformed into 

 spermatozoa, there are generally rather spacious vacant spaces to be 

 found between the sperm masses and the testicular membrane, as well 

 as between each sperm mass (figs. I, 2, 4, 8). Of such spaces, the 

 most spacious one is found in the apical part of the testis, at the base 

 of the sperm-duct (g: s). This space was described by previous writers 

 such as Hertwig ('80) and Garbe ('01) under the name of "Genital- 

 sinus". Judging from the fact that, such spaces are never present in 

 the parts that consist entirely of spermatogonia! cells, but occur invariab- 

 ly in the parts that contain spermatozoa, they seem to develop with 

 the differentiation of the sperm-cells by the concomitant diminution of 

 the volume of the cell masses. 



From the above description it is evident that the male gonad of 

 Coeloplana agrees very strikingly in several respects with that of 

 Ctenoplana described by Willey ('96). The male gonad develops in 

 both genera in certain separate masses more or less compact, instead of 

 series of sexual cells as in ordinary ctenophores. Moreover, each testicular 

 mass is provided with a certain number of sperm-ducts in both of the 

 genera. Differences, however, ma)- be found concerning some rather 

 minor points: — While in Ctenoplana the number of testes is only four in 

 each individual, in Coeloplana it is incompatibly larger; while the number 

 of sperm-ducts belonging to each testis is from one to three in the former, 

 it is always one in the latter. An}-way, such features of the testes, but 

 especially the existence of the sperm-duct, are worthy of special notice, 

 being unknown in any other ctenophore. Mortensen ('12) has entertained 

 doubt on WiLLEv's observation recognizing sperm-ducts in Ctenoplana. 

 Against this, Willey ('13) has contented and advocated his previous 

 view. So far as can be judged from the observation in Coeloplana, 

 Willey's observation seems to have been correct in that respect, as 

 suggested by Krempf also ('21). 



Ovary. 



The ovary of Coeloplana develops as a continuous tract of egg- 

 cells in the dorsal wall of the supharyngeal and subtentacular canals. 



