44 ART. 4. — N. YATSU : 



of the presence of the hinge line, but the secretory activity 

 continues uninterrupted, and thus results the thickening of the 

 shell in this region. In other parts, on the contrary, the mantle 

 increases continually and no special thickening of the shell takes 

 place. 



The microscopical structure of the shell can best be studied 

 in longitudinal sections (PL VIII., Fig. 130.). The shell is 

 composed of cuticular substances secreted in layers. Outside this 

 is a special layer, the periostracum (pr. od.), which measures 

 about 1 j« in thickness, exhibits a slight difference in refractive 

 index from the main cuticular layer, and is of a very light 

 yellow color. 



c. Mantle and Setae. 



It need hardly be remarked that the mantle which gives rise 

 to the shell keeps equal pace with it in growth and therefore 

 corresponds with it in outline. Along the mantle margin there is 

 a thickened zone bearing a yellowish brown pigment (PI. V., 

 Figs. 77-79., and PL VI., Fig. 83.). It is indeed only by this 

 zone, that the presence of the mantle can be recognized, since in 

 other regions it is transparent and invisible. 



Along the margin of the mantle, it should be recalled, a 

 lacuna (Randlacune) is seen in the oldest embryo which was 

 reared from the eggs (PL IV., Fig. 70.) (vide p. 27.). In the 

 larvte of the 5-6 p. c. stage the marginal lacuna is found to be 

 loosely filled with mesenchy matous connective tissue cells which 

 have probably been proliferated from the ectoblast. Later on the 

 lacuna extends toward the body proper. From the 7 p. c. stage 

 on there come into view the muscle fibres (PL VI., Figs. 84, 



