IX 



MOLLUSCA 



357 



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

 shell by mantle Haps is a secondary phenomenon which was not 

 present in the earlier Cephalopoda. 



ym.c. 



me 



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

 stages 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 ; y.m.c, 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, 



FIG. 288. Two early embryos of Loliyv vulgar is. (After Korschelt.) 



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

 side, ar, rudiments of arms ; m.f, mantle-fold ; o.c, optic cup ; o.tt, optic stalk ; s.x, shell-sac ; stvm, 

 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 



