22 AKT. 5. X. YATSU : 



testis in the epithelial ridge of the palliai sinus and mistook the 

 spindle bodies for spermatophores. Moreover he extended his 

 view, regarding the round compact bodies also found in the 

 epithelial ridge as young ova. As I have explained at great 

 length in Section II, these two enigmatical bodies are not sexual 

 elements at all. 



In Lingula the formation of sexual elements is restricted 

 entirely to the ileo-parietal band, and the sexual organs in the 

 palliai sinus found in other Brachiopods are entirely wanting. 

 Both the ova and spermatozoa are nothing else than modified 

 epithelial cells on either side of the ileo-parietal band, as has 

 already been described by many writers on the Testicardines. 



The sexes of Lingula are after some experience easily dis- 

 tinguished from without. Seen through the translucent shell 

 the females appear somewhat dark brown, while the males lighter 

 brown or even whitish. The distinction can best be seen in 

 individuals of a medium size, for in older animals the shell 

 increases in thickness and obscures these features. 



a. Ovary. 



Äs to the structure and development of the ovary : in free- 

 swimming larvae with eight pairs of cirri there are distinctly 

 seen, though few in number, primary germ-cells, 1 some which arc 

 distinguished from other epithelial cells by their larger size, 

 granular cytoplasm, and vesicular nuclei. 



In young Lingula, 4.5 mm. in shell length the sex is already 

 differentiated ; the undifferentiated condition, therefore, must be 

 sought for in individuals younger than this. In PI. II., Fig. 26 



1. Vide N. Yatsu. On the Development of Lingula anatina. This volume, Art. 4, p. GO. 



