18 ART. 5. N. YATSU : 



gastoparietal band, a little posterior to the median half of the 

 occlusor anterior, and near the bottom of the lateral indentation 

 of the body cavity. Fig. 20 (PI. II.) shows the position of the 

 organ. Just distal to this vesicle the palliai sinus comes into 

 communication with the body cavity. 



Generally speaking the organ is lenticular, but there are not 

 wanting variations : in a dorsal view it is sometimes nearly 

 triangular with rounded angles, while in other cases it is almost 

 circular. As a rule it is broader than it is long. Its walls are 

 not of equal thickness, but are thickest on the median and lateral 

 walls, and thinnest on the dorsal and ventral sides. There is 

 also a great variation in size : the following is an average of the 

 measurements of the orsjan in various sections. 



&■ 



Length 



Breadth 



Height 



[thickest portion 

 i thinnest ,, 



Thickness of the walls* 



It 



145 a« 



160/; 



40,« 



35/^ 



8j« 



In tracing a series of longitudinal sections (PI. II., Figs. 21, 

 22) as soon as the valve at the entrance of the palliai sinus 

 comes to an end, and in the same section in which the nephridium 

 is about to open to the exterior on the ventral side the otocyst 

 becomes visible. Following the sections toward the median plane 

 we soon find that the otocyst is a closed sac, as is seen on a 

 surface view, and that it bears not a little resemblance in structure 

 to the same named organ occurring in Arenicola grubii studied 

 by E. Ehlers (Taf. XIII., Fig. 37). It is situated just anterior 

 to the attachment of the gastroparietal band to the dorsal body 

 wall, so that it has a lamella of the supporting substance at the 



