46 ART. 4. — N. YATSU : 



but several planted at the posterolateral corners of tlie mantle 

 are tolerably long, extending for a short distance beyond the 

 shell margin (PL VI., Figs. 84, 85., st.). On a careful study 

 of toto preparations elongated nuclei arranged along a seta seem 

 to support the view that the seta has been secreted by the ecto- 

 dermal invagination as in the adult. This feature is however, 

 very difficult to ascertain in sections. 



d. Palliai Sinus. 



By a careful adjustment of the microscope the palliai sinus 

 can be seen in living larvse as four finger-like processes of the 

 body cavity which stretch into the mantle at the antero -lateral 

 corners of the body proper. They are quite invisible both in 

 fixed larvse and in stained preparations, owing to the fact that in 

 preserved specimens the thin walls of the sinus become closely 

 pressed against each other, so that the lumen is entirely obliterated. 

 They appear at the beginning of the 8 p. c. stage. As shown 

 in Fig. 84. (PL VI., pli. su.) the outline of the sinus appears 

 as an interrupted line. It should be stated that each dot in 

 this line is intended to represent a single flattened cell. Further 

 notes regarding the sinus need not be given, since they only 

 confirm the descriptions of Brooks. 



e. Body Cavity and its Walls. 



In surface view, the outline of the body cavity is circular in 

 most cases, but the diameter changes a great deal according to 

 the degree of contraction of the parietal muscle. Fig. 84. (PL 

 VI.) shows it in a medium sized condition. 



At an early stage the cavity communicates only with the 



