IX 



MOLLUSCA 



357 



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 



yTn.c. 



me 



sBsas*sa5a5?aa?i 



'-Tn.c. 



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

 stages of development ; to illnstrate 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.t 

 layer cells ; y,mx, yollc-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. 



Fig. 288. — Two early embryos of Lolicjo vulgaris. (After Korschelt.) 

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



side, ar, rudiments of arms ; 7ft./, mantle-fold ; 

 stomodaeum. 



ox, optic cup ; o.it, optic stalk ; s.s, shell-siic ; s(om, 



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 



