IX 



MOLLUSCA 



35*7 



the primitive genus Nautilus, we know that this covering in of the 

 shell by mantle flaps is a secondary phenomenon which was not 

 present in the earlier Cephalopoda. 



A 



' rn.c. 



FIG. 287. Two sections through the edge of the blastoderm of Sepia officinalis in different 

 stagi-s of development ; to illustrate the development of the lower layer cells. (After 

 Koeppern, from Minchin's preparations.) 



A, Younger stage. B, Older stage. A is more highly magnified than B. me, mesoderm, i.e. lower 



layer cells ; i/./n.f, yolk-membrane cell. 



The eyes now appear as shallow cups on the sides of lateral 

 protrusions of the body, which may be termed eye-stalks. The edges 

 of the Cephalopod shell gland constitute, as in other Mollusca, the 

 rudiment of the mantle, and underneath them appears a groove, 



FlO. 288. Two early embryos of Lufii/u ruli/n ris. (After Korselielt.) 



A, An embryo seen from the posterior side. B, A slightly older embryo >een IVoni the anterior 

 side, ar, rudiments of anus; HI./, mantle-fold ; o.c, optic cup; o.it, optic stalk; ...-, shell-sac ; stom, 

 stomodaeum. 



deepest behind, which is the rudiment of the mantle-cavity. Just 



within this groove two buds appear to the right and left of the 



middle line ; these are the rudiments of the gills : whilst below them 



appear two pairs of ridges converging towards the ventral surface and 



